The Queen, members of her family and the Prime Minister are attending events to mark the 800th anniversary of the signing of the Magna Carta.
Commemorations are taking place at Runnymede, the site where the treaty between a tyrannical king and his rebellious barons was agreed.
Translated in English as Great Charter, it was signed in June 1215, but within weeks it was torn up and the country plunged into civil war.
Despite this, many of the principles in the charter survived and became law - with the language adopted in democracies around the world.
A statue of Her Majesty Queen Elizabeth was unveiled on the site on the banks of the River Thames on Sunday.
Her predecessor, King John, is regarded as one of Britain's worst monarchs, and it was his dispute with the landowning aristocracy which formed the background to the creation of Magna Carta.
The barons forced him to accept new laws and a limitation on his power.
There are four copies of the charter still in existence - one each in Lincoln and Salisbury Cathedrals, and two in the British Library.
The curator of the Library's exhibit, Dr Claire Breay, told Sky News: "The most important thing about Magna Carta is that it established the principle of the rule of law.
"No free man shall be seized or imprisoned or stripped of his rights, or outlawed or exiled, except by the judgement of his equals or by the law of the land.
"And that clause is really at the heart of Magna Carta's fame today."
Those who negotiated the treaty would be astonished at how its reputation has survived eight centuries, because it was annulled after only 10 weeks.
The Pope ruled that King John had been forced to sign it under duress.
Yet in the years afterwards, the language in the charter was revised and reintroduced and became part of the cornerstone of English law.
"If the barons looked at how we were celebrating it they'd be quite amused," says human rights barrister John Cooper QC.
He equates Magna Carta to scoring an early goal in a football match.
It wasn't decisive, but it shaped what followed.
And he argues that some of the rights envisaged by the charter, such as trial by jury, are under threat.
While David Cameron will take part in the anniversary events , he has also advocated the abolition of the modern Human Rights Act, and withdrawing Britain from the obligations of the European Convention on Human Rights.
His former attorney general Dominic Grieve, who's been heavily involved in Magna Carta events, disagrees with him.
"There is something that we need to learn from the charter, which is that if you want other people to respect rights, you yourself have got to respect those rights," he said.
"I happen to believe that although it's not perfect, both the European Convention and the Human Rights Act are working pretty well."