Winston Churchill's plans to give the US a copy of Magna Carta, in return for its support in World War Two, are revealed as part of a new exhibition.
Government papers revealing Churchill's part in the bid to boost US support are on display at the British Library.
Also on display for the first time in the UK are the US Declaration of Independence and the US Bill of Rights, alongside two Magna Carta manuscripts.
Magna Carta: Law, Liberty, Legacy marks the manuscript's 800th anniversary.
Magna Carta, authorised on 15 June 1215, is considered one of the first steps towards parliamentary democracy. It included the principle that no one was above the law, including the King.
As World War Two broke out, one of the original copies was stranded in America, following its display at a trade fair in New York.
Foreign Office papers now on display at the British Library show the Cabinet contemplated giving the manuscript to the United States as a gift, describing it as "the only really adequate gesture which it is in our power to make in return for the means to preserve our country."
Winston Churchill had added handwritten comments in favour of the proposal but it fell through as the Magna Carta was not the government's to give away.
It actually belonged to Lincoln Cathedral and for the rest of the war it was guarded at Fort Knox before being returned in 1946.
More than 200 items are on display in Magna Carta: Law, Liberty, Legacy.
They include King John's teeth and thumb bone and 800 year old clothes and crozier belonging to the Archbishop of Canterbury at the time, Archbishop Walter.
The portrait of Herbert Beerbohm Tree as King John in King John by William Shakespeare (above left) is by Charles Buchel (1900) and is on loan from the Victoria & Albert Museum.
The statue of Geoffrey de Mandeville (above right), one of the Barons, is usually in the chamber of the House of Lords.
A replica of King John's tomb (above) usually resides in Worcester Cathedral.
Two of the most iconic documents in American history, Thomas Jefferson's handwritten copy of the Declaration of Independence and the Delaware copy of the US Bill of Rights (above) are in the UK for the first time. Both were inspired by Magna Carta.
The exhibition also includes two molars and a thumb bone (above) belonging to medieval monarch King John, who granted the charter of the Magna Carta. They are on display alongside his original will and fragments of clothing taken from the King's tomb in 1797, when it was opened to verify that the King was buried there.
Prominent politicians, historians and public figures including Aung San Suu Kyi, Bill Clinton and William Hague have given interviews on what Magna Carta means today, which will feature in the exhibition, alongside a recent Horrible Histories Magna Carta 'rap battle' between King John and the Barons.
Magna Carta: Law, Liberty, Legacy opens on Friday 13 March and runs until 1 September 2015 at the British Library.