A rare handwritten copy of the US Declaration of Independence - thought to be only the second in existence - has been found in Britain.

Hidden at the West Sussex Record Office in Chichester, it was found by two researchers compiling records for a database, and has been dubbed the Sussex Declaration.

The original, signed in Philadelphia on July 4 1776 and on display at the National Archives in Washington, proclaimed that the 13 American colonies were free from British rule.

Harvard researchers Emily Sneff and Danielle Allen discovered the 24in by 30in parchment, which is the same size as the original but written horizontally.

They believe it was created in the US in the 1780s.

Federalist and Supreme Court Justice James Wilson, one of six men to sign both the Declaration and the Constitution, is thought to be the "likeliest candidate" to have created the copy.

He was known as a supporter of the argument that the US was founded by its people and not a confederation of states.

Professor Allen said: "Up until now only one large format ceremonial parchment manuscript was known to exist. That one is in the National Archives and was produced in 1776.

"This one was produced a decade later with the signed parchment as its source, as part of the fight between federalists and anti-federalists about whether the new republic was founded on the authority of a single, united sovereign people or on the authority of 13 separate state governments.

"The federalists were making the first argument and this document appears to have been produced to support their case. It illuminates the politics of the 1780s in a flash."

The researchers are now attempting to establish exactly how the Sussex Declaration ended up in England, but believe it could have been held by the Third Duke of Richmond, Charles Lennox, as it was left at the Record Office with other papers from his law firm.

The researchers' report said: "While the parchment may have moved to the UK in the 1780s or 1790s, when the Third Duke could have received it, it is also possible that it moved to the UK only after 1836."