Fundamental reforms to UK human rights laws are required to "safeguard the legacy" of Magna Carta, David Cameron will declare today as he joins a ceremony to celebrate the 800th anniversary of the groundbreaking accord.
The Prime Minister will join the Queen, senior members of the Royal Family and the Archbishop of Canterbury at Runnymede, where King John accepted the historic document that limited the power of the Crown on June 15 1215, to mark the occasion in front of thousands of spectators.
He will say its principle is "as relevant today as it was then" and remains "sewn into the fabric of our nation, so deep we barely even question it," but will complain that the notion of human rights in Britain eight centuries on has been "distorted and devalued".
The Conservative government has controversial plans to scrap the Human Rights Act and assert the supremacy of the UK's supreme court over the European Court of Justice in Strasbourg - leaving open the option of withdrawing from the European Convention of Human Rights if reforms are blocked.
"It falls to us in this generation to restore the reputation of those rights – and their critical underpinning of our legal system," he is due to declare in a speech to guests.
"It is our duty to safeguard the legacy, the idea, the momentous achievement of those barons. And there couldn't be a better time to reaffirm that commitment than on an anniversary like this.
"So on this historic day, let's pledge to keep those principles alight."
The Queen, joined by the Duke of Edinburgh, will lead commemorations in her role as monarch and also as patron of the Magna Carta Trust, which has organised a series of anniversary events throughout the year.
Today's event will include speeches by the Archbishop of Canterbury Justin Welby and Lord Dyson, Master of the Rolls, musical performances and the rededication of the American Bar Association's Magna Carta Memorial by the Princess Royal.
The Queen will unveil a plaque to mark the occasion and a new installation by artist Hew Locke will also be unveiled on the site by the Duke of Cambridge.
Although just three of its clauses remain law in the UK, the Magna Carta set a precedent that saw it influence later works domestically and abroad, including the US Constitution and the Bill of Rights, and the post Second World War UN Universal Declaration of Human Rights.
Mr Cameron will say Magna Carta had been "revolutionary - altering forever the balance of power between the governed and the government."
Down the years it had inspired the fighters in the English Civil War, the Chartists, the Suffragettes, the founders of the first American states, Gandhi, Nelson Mandela and "anyone challenging injustice or checking arbitrary power", he will add.
"Magna Carta takes on further relevance today. For centuries, it has been quoted to help promote human rights and alleviate suffering all around the world.
"But here in Britain ironically, the place where those ideas were first set out, the good name of 'human rights' has sometimes become distorted and devalued."
Though more later versions remain, just four known copies of the original Magna Carta (Great Charter) exist today, from an estimated 13 that were made.
Two are held by the British Library, with Salisbury Cathedral and Lincoln Cathedral holding the others. They are written in Latin on sheepskin.
Urging everyone in Britain to take pride in Magna Carta, Mr Cameron will say: "Its remaining copies may be faded, but its principles shine as brightly as ever in every courtroom and every classroom from palace to Parliament to parish church.
"Liberty, justice, democracy, the rule of law – we hold these things dear and we should hold them even dearer for the fact that they took shape right here, on the banks of the Thames.
"So on this historic day, let's pledge to keep those principles alight. Let's keep Magna Carta alive.
"Because as those barons showed, all those years ago, what we do today will shape the world for many, many years to come."