The prime minister's wife, Cherie Blair, has been ranked the 12th most powerful woman in the world by business magazine Forbes.
Mrs Blair gets power from her work and her husband, Forbes said.
She was one of six UK women to make the top 100 list, led by US National Security Adviser Condoleezza Rice.
Former prime minister Margaret Thatcher was ranked at 21, followed by the Queen at 22 and author JK Rowling at 85.
Pearson chief executive Marjorie Scardino and London Stock Exchange boss Clara Furse also made the list.
The top three most powerful women were: Ms Rice; China's health minister and a vice-premier, Wu Yi; and the president of India's Congress Party, Sonia Gandhi.
President Bush's wife, Laura, was placed fourth, followed by her predecessor as First Lady, Hillary Clinton at fifth place.
The magazine described Mrs Blair as getting "her power both from her own work and from that of her husband".
It noted that not only does she live at No 10 Downing Street as "Britain's first lady", she also practices under her maiden name as a Queen's Counsel with London's Matrix Chambers law firm.
"Today she's helping her husband behind the scenes, as Britain continues to weather the storms about its alliance with the US over Iraq," the magazine said.
Baroness Margaret Thatcher, prime minister from 1979 to 1990, was described as "brilliant and gifted with an electrifying, withering wit, Thatcher's is the voice of clear-eyed realism."
The magazine highlighted her role - "along with her friend Ronald Reagan" - in the collapse of the Soviet Union and the push for "a worldwide embrace of free markets".
JK Rowling, author of the Harry Potter series, has sold 250 million books and "become a billionaire in the process", the magazine said.
The Queen was not the only royalty on the list: Queen Rania of Jordan was the most powerful queen, ranked 13.
Queen Elizabeth came next at 22, followed by Queen Beatrix of the Netherlands at 35, Queen Sofia of Spain at 39 and Queen Silvia of Sweden at 68.
Marjorie Scardino, chief executive of Pearson which publishes the Financial Times and Penguin, was ranked 59th.
The chief executive of the London Stock Exchange, Clara Furse, narrowly made the list, ranked at 97.
Who is the most powerful and influential woman in the world? Read some of your views.
The so-called top 100 women in the world have almost no influence on my life. My list would have my wife Alda as number one.
John Austin, Princeton, NJ, USA
The most powerful lady in the world has to be Laura Bush. She has the ear, and can influence, by far the most powerful man in the world.
Condoleezza Rice may be the most powerful according to Forbes, but my mum would win over all of them.
Dean, Plymouth, Devon
My mother and my sister. I love them both very much.
Petr Matyas, Prerov, Czech Republic
For me, Condoleezza Rice is the most powerful. She is the closest adviser of President Bush who heads the USA. With this, she has a great influence in US policy. This can be proven through the Iraqi issue. Aside from that, her credibility and experience in foreign policy made her the person that Mr Bush consults on delicate matters regarding the US.
Ersa , Philippines
Very few of them seem to have achieved their "power" due to their own hard work. Either they were born into insane wealth, or married well.
Judy Hipkiss, Coventry
You must be kidding me if Oprah Winfrey is not in the list of the most powerful woman in the US. Laura Bush and Hillary Clinton do not even come close to her knee. Do me a favour, make sure you check your facts before you put them up there... Thanks
Franck, Silver spring, Maryland, USA
Mother Theresa of course! Had no army, military, air force or husband (in power, note to Cherie Blair). Her belief and deeds gave her her (spiritual) power.
Chux, London, UK
The school crossing lady outside the feeder school to Eton College
David Peacock, Romford, UK
Being an Indian I do agree and feel proud that Sonia Gandhi's name is in the 3rd place of the most powerful women in the world. This is a real fact as she is very innocent to her work, and is not power-hungry. She is a woman of substance. Although she is not the prime minister, she cares for everyone and I know that under her leadership our country will be much more powerful and in safer hands.
Chetan Advani, Mumbai, India
The most powerful woman and most influential in the world according to me will always be Oprah Winfrey. This woman has come far and, to be what she is now, would take very few to accomplish. The woman came literally from an abusive home and look at her now. She has touched many people's lives and the recent one was when she turned a young woman known as Lashee Griffin into a star. That woman inspires not only the young but the old too. She deserves to be rated as the best in the world. Please let's always learn to give credit were it's due.
Millicent Richards, Kampala, Uganda
Oprah Winfrey is most certainly the most influential person in the US and even perhaps in the world. So many women there and elsewhere pay attention to her and her beliefs.
John Kelly, Dublin, Ireland
Condoleezza Rice. I have a lot of respect for the pressure and the appointment of a female to such a challenging position. Especially considering the trial of the September 11 event and inquiry, she has proven worthy of higher calling. Despite this, she is the only female appointed for such hard duty.
Mofu Chisulo, Lusaka, Zambia, Central Africa
Any wife or girlfriend has to be the most powerful woman in the world.
The most powerful woman should be the lady who sells fried banana for tea every evening downstairs the block where I work.
Abd Rahman, Kuala Lumpur, Malaysia
Victoria Beckham, because she has such a powerful influence on peoples emotions - mostly adverse. Power is only perceived by its effect on the individual, Cherie Blair does not have a visible effect on people's lives.
Cliff Wood, UK
I wouldn't say that HM Queen Elizabeth II is the most powerful woman in the world but I completely disagree with her position at 22. I think she would be much higher in the rankings and believe she should top all British women as I personally think that she has by far the most influence on the country.
Richard Booth, Durham, UK
By ranking a woman who isn't in an actual post or position herself but derives her ranking via her husband's position is pure conjecture. It also diminishes the ranking of those women who have made it on their own merits. How can one even think of comparing Margaret Thatcher and Laura Bush, for example ?
David, Simonstown South Africa
It's sad to see a list where so many of the women are only there because of accidents of birth or because they have gained prominence due to who they married.
Mike, Pontypridd, UK
It is Mary Robinson because she stands for human values, held high office in the UN, and she is a very clear minded, courageous leader.
Masika Yao, Sudan
The Queen. If she were to say something was not being done when it should have been....the world would sit up and listen.
James O'Shea, New Ross, Ireland
The most powerful woman is Laura Bush. She has the same merits as Cherie Blair. I know of hundreds of lawyers who will never appear in that list because they didn't marry the right man.
Ghilberto, Paris, France
The Queen of England. When the Queen speaks, people listen. Although the government are trying to disband the Royal family, the public have more respect for the Queen than any member of Parliament. Whoever ranked Cheri Blair 12th needs to live in the UK for a while and witness the UK's view of Cheri. I feel that all she is, is the wife of a so-called leader who is failing his country.
George Henry, Swansea
I'd have thought that if one was talking about the "first Lady" of the UK, one would be referring to HM The Queen, rather than Cherie Blair.
Andy Jackson, Leicester, UK
My mother is indeed the most powerful...born in a small village in Africa and despite not having a college degree, she raised and educated me enough not only to enter the top school in the world - Harvard University, but also to be accepted into a program at Oxford which I thoroughly enjoyed!
Siza Mtimbiri, Merrimack, USA
Mrs Blair is not first lady any more than Bliar is head of state. Someone should have told the magazine and the BBC should have made the point itself in the above! She also should have no power whatever, she has no elected position, and mere lawyers are not in the business of power.
James StGeorge, London England
Dr. Rice is no doubt the top powerful lady in the world. I think Begum Zia or Sheikh Hasina of Bangladesh should be at least within the first 20 women of the world.
Tareq Chowdhury, Sylhet, Bangladesh
Sonia Gandhi, undoubtedly the most powerful and loved by most. Sonia is not only loved by Indians but globally.
Forbes, an American magazine, may have selected Condoleezza Rice as she belongs to the Cowboy culture. Condoleezza's approach is more like a dictator and less justified like the Iraq invasion and occupation.
Siva, Toronto, Canada
I think my mother is the most powerful woman in the world. That's my view. I have never met any of these so-called powerful women and I have never experienced in any way or shape, their 'power'. But as I was growing up I could feel my mother's power running down the entire family hierarchy. She remains powerful to me. What a source of inspiration she has been to us!
Phildah Chihuri, Harare, Zimbabwe
The most powerful and influential woman in the world is one who has moral credulity and love for humanity and fights for justice and peace in society and the world.
Munir Othman, Kosovo
The Dixie Chicks and the Spice Girls in a dead heat., West Palm Beach, Florida, USA
Clearly Barbara Walters is THE global dominatrix. I can't believe she didn't top this list.
Darryl, New York
I disagree with the criteria, powerful and influential are not compatible. HM the Queen is certainly more influential but powerful she is not. Likewise, Dr Condoleezza Rice is powerful but not influential. And Ms Oprah Winfrey could perhaps claim both; she has money and she is popular.
Jimmy Greenshield, Savannah, USA
To me, my mother (Samar) is the most amazing woman in the world. After her, I truly believe that Queen Rania of Jordan is the most powerful and influential woman in the world.
Basil , New York City, USA
The survey seems to be based on the criteria that considers political power and not the 'influence' factors. I think, if seen in real perspective, Benazir Bhutto is the most influential person. The Musharraf regime in Pakistan is not scared of Al- Qaeda terrorists. But if it us scared of anything at all, then it is Benazir Bhutto. Undemocratic regime is OK for USA and the British Commonwealth, but not a democratic system which, rightly or wrongly, is now represented in Pakistan by Benazir Bhutto.
Azra Imam, Sutton, UK