David Cameron champions Magna Carta but in 2012 he didn't know what it meant when he appeared on David Letterman's Late Show

David Cameron showered the Magna Carta with praise today on its 800th anniversary, but just three years ago he didn't know what it meant.

The Prime Minister embarrassed himself live on the world's most famous late night TV show, giving a number of incorrect answers as David Letterman quizzed his knowledge of core aspects of Britain's history on the Late Show in 2012.

Asked for the literal translation of 'Magna Carta,' Mr Cameron was stumped, failing even to take a guess. “Oh it would be good if you knew this," Mr Letterman joked. "It literally means 'Great Charter,'" the famous presenter informed him.

Quizzed about its whereabouts now, Mr Cameron could only manage to say "it does exist." And he incorrectly guessed who composed Rule Britannia, taking a stab at Elgar. Mr Letterman told curious viewers that it was in fact Thomas Arne.

Admitting his failure at the end of the questioning, Mr Cameron said: "You have found me out. That is bad, I have ended my career on your show tonight," he joked.

His old friend and rival Boris Johnson jumped to his defence, claiming Mr Cameron had deliberately fluffed his lines so he didn't appear too clever. "I think he was only pretending," he suggested. "I think he knew full well what Magna Carta means.

"It was a brilliant move in order to show his demotic credentials and that he didn't have Latin bursting out of every orifice," added the Mayor of London, who takes every chance to show off his own knowledge of Latin.

Mr Cameron, who became the first sitting British Prime Minister to appear on the Late Show, was welcomed onto the stage to the tune of Rule Britannia and dry ice pumping into the studio to replicate a London fog.

Mr Cameron used the 800th anniversary of the charter today to champion his government's bid to replace the Human Rights Act with a British Bill of Rights. He argued Labour "devalued" the legacy of the Magna Carta in Britain's legal system as he promised to "restore" its status by scrapping the HRA.

The Magna Carta was signed by King John in 1215, giving his subjects rights, protections and security for the first time.

Mr Cameron has not ruled out pulling Britain out of the European Convention on Human Rights, which Britain signed up to in the 1950s but was only enshrined into law by Tony Blair in 1998.

Yvette Cooper, Labour's shadow home secretary, accused Mr Cameron of trying to "hijack" the charter to push his own agenda to abolish the HRA.

“The Prime Minister is trying to hijack this important celebration of the Magna Carta to push his ill-thought through plans for abolishing the Human Rights Act. It demeans his office," she said.

"The Magna Carta is rightly seen by people across the world as the historic foundation of our democratic rights. Some of it remains in legislation, including the right to justice and a fair trial. It's a wonderful thing and it's right that we mark its 800th anniversary.

"The Prime Minister should be proud of spreading our historic human rights tradition across Europe and the world, rather than trying to rip it up," she added.

Last year Mr Cameron was mocked when he unveiled plans to teach all school children about Magna Carta as part of the government's bid to make schools "more muscular" in promoting British values. Critics pointed to his own ignorance of the Magna Carta, suggesting he should brush up on his knowledge before telling others to.