The secrets of Parliament's Victoria Tower uncovered
Some of the three million documents held at the Victoria Tower including Charles I's death warrant
Millions of tourists photograph the Houses of Parliament every year, in particular, the Clock Tower (also known as Big Ben) but who knows anything about the tower at the other end?
BBC London's Asad Ahmad takes a tour around the Victoria Tower.
London has some of the most famous buildings in the world.
A short walk through central London, even for someone making their first trip to the capital, is like walking through a magnificent book of history where the Tower of London, Buckingham Palace and Tower Bridge all come to life.
Standing as one of the finest works of architecture in the capital is the Houses of Parliament designed by Sir Charles Barry in the 19th century.
His plans were drawn-up to enter a competition to create a new Parliament building after the previous one had been destroyed by fire in 1834.
To look through some of the others would be like accessing a historians Aladdin's Cave but that is probably why the secrets of the Tower are kept safely from curious eyes.
The new 'Mother of all Parliaments' at Westminster has since become an emblem for democracy all over the world.
As tourists and Londoners snap pictures of the more famous Big Ben/Clock Tower, the Victoria Tower stands quietly on the other side of Parliament.
This year, the 323ft (98.5m) structure has been celebrating its 150th birthday.
When it was constructed, it was the largest square tower anywhere in the world. It no longer holds that particular record, but it does hold millions of other records within its walls.
The Victoria Tower was designed to hold papers related to Parliament which go back to 1497 and the reign of King Henry VII.
All documents are original and, as well as holding every Act of Parliament to be passed in Britain, there are all sorts of objects and papers which chart every twist and turn of British history over 500 years.
Due to its unique contents very few people have ever been given access to the tower, although it has been described as one of the most historically important places in the world.
David Prior helps to run the Parliamentary Archives Office which takes day-to-day responsibility for the tower and its contents.
He knows its importance more than anyone and he even explains how the air conditioning system, which helps to regulate the temperature on all 12 floors, is environmentally friendly.
The Victoria Tower in Westminster
"We take our responsibility for the records of Parliament very seriously," he said.
"But we are also aware of our responsibility to the environment."
It is important to regulate the temperature and air inside to protect the documents and Mr Prior shows me some of the papers that have rarely been seen by the public.
"One of our most famous documents is the death warrant of Charles I," he said.
"It's the only time a ruling British monarch has stood trial, before being executed.
"If you look at the signatures under the warrant and their wax seals, you notice the fourth signature, it belongs to Oliver Cromwell."
It is incredible to see the death warrant of a former British King signed by Oliver Cromwell.
I realise that if the Victoria Tower was a museum it would probably be the ultimate museum.
David Prior then points to the Stamp Act of 1765. It is made of animal skins which have been sewn together, but its importance is huge.
The Stamp Act was passed to make America pay duties and taxes to their British rulers.
Not surprisingly, the Act was not popular across the Atlantic and David said: "The Act was partly responsible for beginning the movement which fought the British and ultimately brought America its independence from Britain".
Sandwiched between the death warrant and the Stamp Act is a journal which is dated 6 November 1605.
The first Act of Parliament signed by King Henry VII
"If you look closely, it's written that somebody by the name of John Johnson, was found with 36 barrels of gunpowder in Parliament the previous night." David points to the relevant section. "John Johnson," he tells me "is the alias for Guy Fawkes."
The journal leaves me speechless although these are just three of three million documents kept in the Tower over the past 150 years.
To look through some of the others would be like accessing a historian's Aladdin's Cave but that is probably why the secrets of the Tower are kept safely from curious eyes.
It is true the Tower does not get much attention, but that is probably the way the keepers of the documents want things to remain - even if they are celebrating 150 years of keeping some of the contents of London's most important and striking buildings - a secret.
The House of Lords Parliament Archives Department have published a book to mark 150 years at the Victoria Tower. 'Victoria Tower Treasures' is available from the department.