As Northern Ireland Secretary, Mo Mowlam oversaw the negotiations which led to the 1998 Good Friday Agreement.
Leading political figures have been giving their reaction to her death.
PETER HAIN, NORTHERN IRELAND SECRETARY
She was the catalyst that allowed politics to move forward which led to the signing of the Good Friday Agreement in April 1998.
She cut through conventions and made difficult decisions that gave momentum to political progress.
Mr Hain paid tribute to Mo Mowlam's visit to the Maze
I remember, particularly, her courageous decision to meet with loyalist prisoners in the Maze in January 1998 who had threatened to walk away from the peace process - it was personal interventions such as this that made the difference and kept politics moving in what was a very difficult environment.
I spoke to her only three weeks ago after the IRA statement and she told me how significant it was.
I firmly believe that Northern Ireland is a better place for Mo Mowlam having been there.
IAN PAISLEY, DEMOCRATIC UNIONIST LEADER
Her battle against illness was faced with bravery and determination and amidst all her health problems she retained her character and personality.
She was, of course, no lover of unionists. Stating that convicted murders were unsung heroes of the peace process caused great offence.
Nevertheless, she did recognise those who were truly unionist, but sadly never faced up to the widespread opposition to the Belfast Agreement.
When the full story of recent years is written one will see just how far the Labour government was prepared to go in appeasing terrorists and thugs.
The unionist cause will continue to achieve its lawful objectives, but my thoughts today are with Mo Mowlam's husband and family.
MARTIN MCGUINNESS, SINN FEIN
She played a crucial role in the negotiations which led to the Good Friday Agreement and it is that Agreement which will be remembered as her political legacy.
We were, of course, always conscious of the fact that she was a British minister and was at the mercy of the securocrats within the Northern Ireland Office system.
Mr McGuinness acknowledged they had their differences
There were, of course, occasions when we clashed, most notably when she caved in to Orange Order threats and forced an Orange march along the Garvaghy Road in Portadown and when she authorised the bugging of a car used by Gerry Adams and myself during the negotiations.
However, Mo Mowlam will be remembered fondly by the vast majority of Irish people and I would extend my thoughts and sympathies to her husband Jon and the rest of her family at this time.
MARK DURKAN, SDLP LEADER
Mo had a natural feel for this place and in many ways had her own constituency here, so when she came to office she had more than what other secretaries of state had.
She had a strong sense of what needed to be done to get things moving fully again and she followed through on that.
We all talk about her buzz and her touchy feely approach. She didn't take herself seriously but she took her responsibility seriously.
SIR REG EMPEY, ULSTER UNIONIST LEADER
She had a rapport with the general public - she wasn't a stuffy, suited politician, but a real person and she came across very much that way.
It was clear when you were with her over a long period of time, as the day wore on, her stamina wasn't what it might have been.
Sir Reg Empey said Mo Mowlam was not stuffy
I think we underestimated the degree to which she was suffering at that stage.
I think though that's one of the reasons why she was so popular with the general public - she wasn't the archetypal Whitehall politician.
JEFFREY DONALDSON, DUP
She came to visit unionist headquarters in Belfast, and seated around the table were the party officers. Mo came in and within five minutes, she had her feet up on the table and was resting back on the chair and this was an introduction to a very unconventional politician.
I suppose we in Northern Ireland had been used to secretaries of state who sometimes adopted a very patrician, almost colonial style, approach to Northern Ireland - a kind of governor approach.
And yet here was someone who had a very different style - very laid back, very easy manner, easy to talk to.
And while of course we had many differences with Mo Mowlam, certainly she was someone who left an indelible mark on the politics of Northern Ireland.
DAVID TRIMBLE, FORMER ULSTER UNIONIST LEADER
I had no problems with her approach, in fact actually, I welcomed the directness of her approach.
We had too many experiences in Northern Ireland with people who say one thing and mean another, and it was... like a breath of fresh air dealing with Mo, because it was clear where you were and what she wanted.
Mr Trimble said Mo Mowlam was a refreshing change
And you could have a good argument with her one day and have good relations with her the next day, and that's precisely what I wanted.
JOHN HUME, FORMER SDLP LEADER
Her primary objective, of course, for Northern Ireland, was peace and justice and, coming amongst the people, she understood the problem thoroughly.
(She) was right to the forefront in the Good Friday Agreement.
John Hume said she was a key figure in the Agreement
The very fact that, for the first time in history, the people of Ireland - north and south - overwhelmingly voted as to how they wished to live together by voting for the Good Friday Agreement is an enormous tribute to her.
It's our duty to implement the will of the people and to pay our real tribute to her.
MARY McALEESE, IRISH PRESIDENT
She brought great spirit and courage to all aspects of her life, both
professional and personal and will be sadly missed by all those who knew and
worked with her.
Even after leaving office, Mo Mowlam continued to express in so many ways
her deep affection for the people of Ireland and I know this affection was
returned a hundredfold.
BERTIE AHERN, IRISH PRIME MINISTER
Even at her lowest moments, she always seemed to have enough energy and enthusiasm to lift an occasion and to inspire those around her. No matter what the challenge, Mo tackled it with courage and sincerity.
She was a politician and a person whom the Irish people held in great affection and esteem.
Mo Mowlam worked tirelessly in the negotiations for the Good Friday Agreement. As secretary of state for Northern Ireland she was prepared to take risks for the peace process - risks to secure agreement and risks to implement it.
If politics is about securing change for the common good, the peace and prosperity we enjoy today are a measure of how Mo's sense of public service has transformed the lives of people across these islands.
BILL CLINTON, FORMER US PRESIDENT
Hillary and I are saddened by the death of Marjorie Mowlam.
Bill Clinton said she was an integral part of the peace process
Mo was an integral part of building a peace process in Northern Ireland that has endured for over a decade.
Her persistence, toughness and good humour were legendary. All of us who worked to support peace in Northern Ireland owe her our gratitude.
Hillary and I cherished the times we spent together and will carry the warmth of her friendship always.
GEORGE MITCHELL, FORMER US SENATOR
Mo Mowlam made a major contribution to the peace process in Northern Ireland at a crucial time when little progress was being made.
She contributed energy, enthusiasm and optimism. She helped reinvigorate the process and bring it to a successful conclusion.
She was an outstanding public official, a warm human being, very colourful.
She was a good personal friend to me. I came to know and admire her personally. I am very sad indeed at her passing.
DAVID FORD, ALLIANCE PARTY LEADER
Both as shadow secretary of state before 1997 and then in office following that election, Mo Mowlam was a breath of fresh air for Northern Ireland.
She did not get on well with some of the political class but her warm personality engaged with the general public in a way that few from Westminster have ever done.
This was important in ensuring the public pressure on politicians to reach agreement.
Government needs personalities like Mo. She will be greatly missed.
MONICA MCWILLIAMS, WOMEN'S COALITION
Obviously there were different views towards her, but Mo worked very, very hard to win the support of both communities.
When she made mistakes she acknowledged it straight away and apologised - which was so unusual in a politician. That marked her out as unique.
Monica McWilliams said Mo Mowlam was an inspiration
She loved Northern Ireland and was committed and dedicated to the province. She committed herself to making the Agreement to the point where if anybody's hand was on it, it was Mo's.
I also think the way she battled her illness and didn't let it get in her way was an inspiration not just to women but to everyone.
DAVID ERVINE, PROGRESSIVE UNIONIST PARTY LEADER
I may not have always agreed with her, but today I really feel I've lost a personal friend.
Where others before and since tiptoed around us, Mo Mowlam wore hobnail boots. You could disagree with her and argue she did things wrong, but you had to acknowledge one great attribute as a politician - she was someone who really cared.
PUP leader David Ervine said she was 'no slave to convention'
Mo made some men nervous. I know in the PUP, men liked her but women loved her.
In that respect, she made a real difference in Northern Ireland. Here was a woman who was no slave to convention.
DAVID LIDINGTON, CONSERVATIVE NI SPOKESMAN
Her brave and very public battle against cancer gave hope and inspiration to many other sufferers.
She devoted herself to the search for peace in Northern Ireland with both energy and passion.
Some of her decisions were, and remain, controversial, but there can be no doubt that she acted always with the highest of motives.
The greatest and best tribute to Mo Mowlam would be if the political process in Northern Ireland were now to conclude in the just and enduring peace that she sought.
LEMBIT OPIK, LIBERAL DEMOCRAT NI SPOKESMAN
I always genuinely enjoyed working with Mo. She was a warm, humane person with a wonderful sense of humour.
She was also a brave, intelligent, and principled politician. History will look back on her as a hugely influential figure in the story of the Northern Ireland peace process.
It was a pleasure and an honour to work with her.