LONDON (REUTERS) - US President Donald Trump was shown a handwritten parchment copy of the US Declaration of Independence - one of only two known to exist - during a visit to British Prime Minister Theresa May's office on Tuesday (June 4).

Mr Trump, on a three-day state visit to London with his wife Melania, viewed the document with Mrs May and her husband Philip ahead of talks at her Downing Street residence.

The manuscript, known as the Sussex Declaration, is one of only two known handwritten parchment copies of America's formative text.

It had been stored for more than 60 years in a strongroom among miles of documents in the West Sussex Record Office in southern England, until its significance was revealed by two Harvard University researchers last year.

Mrs May's office said it hoped arrangements could be made for the document to form part of commemorations in 2026 marking 250 years since the Declaration of Independence. This could include its loan to the United States, it said.

Measuring 60cm by 76cm, the Sussex Declaration is thought to date to the 1780s and most likely was written in New York or Philadelphia.

While other copies and printed versions of the Declaration exist, the only other ceremonial parchment is the Matlack Declaration, which dates from 1776 and is kept at the National Archives in Washington.

Mrs May will also present Mr Trump with a reminder of close Anglo-American ties on Tuesday, marking his state visit to Britain with a copy of a 1941 charter that helped define the post-World War II global order.

The gift is a framed reproduction of the Atlantic Charter which hung on wartime prime minister Winston Churchill's wall and set out principles of free trade and collective security that formed the basis of the post-war peace.

The draft document setting out policies on which both countries would "base their hopes for a better future of the world" is scrawled with Sir Churchill's annotations, barely legible in faded red pen. It was agreed by Sir Churchill and US president Franklin Roosevelt.

Mrs May's choice of gift underlines Britain's desire to secure a new relationship with the United States for the post-Brexit era, as London quits the European Union and looks to sustain tight security ties with Washington and lock down a new trade deal.

During Mr Trump's presidency, the alliance with Britain - famously nurtured by US president Ronald Reagan and prime minister Margaret Thatcher in the 1980s - is increasingly fraying.