Washington believes that the ‘special relationship’ between Britain and the US is over, according to a secret briefing document seen by The Mail on Sunday.

The memo for members of Congress states damningly that ‘the UK may not be viewed as centrally relevant to the United States in all of the issues and relations considered a priority on the US agenda’.

Dated April 2015 and drawn up to brief the Senate and House of Representatives on the impact of Britain’s General Election, the memo also warns that the UK faces turmoil if there is a hung parliament.

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The 'special relationship' between Britain and the US was strong under Margaret Thatcher and Ronald Reagan

The document – prepared by the Congressional Research Service, an in-house intelligence body that gives confidential analysis to legislators – states that while Britain and the US are likely to ‘remain key economic partners’, a ‘reassessment of the special relationship may be in order… because its geopolitical setting has been changing’.

The memo, edited by Derek E Mix, the CRS’s chief European affairs analyst, says that the development of organisations such as the G20 group of major economies has led to a decline in the ‘influence and centrality of the relationship’.

It also states that the UK’s continued importance to the US will hinge on the future success of the economy – and Chancellor George Osborne’s implementation of spending cuts.

It reads: ‘A significant degree of the UK’s international influence flows from the success and dynamism of the British economy, further raising the stakes on whether the UK can sustain stronger economic growth while continuing to pursue ambitious fiscal consolidation.’

Prime Minister David Cameron and US President Barack Obama serve food during a Downing Street barbecue in May 2011. When Cameron visited the White House in January, he insisted the President had said the special relationship was ‘stronger than it has ever been’

Winston Churchill first coined the phrase 'special relationship' in a speech in 1944

The ‘special relationship’ has been deployed by generations of politicians – most notably Ronald Reagan and Margaret Thatcher – to describe the close political, diplomatic, cultural, economic, military and historical relations between the two countries.

It was first coined in a 1944 speech by Winston Churchill, when he said it was his ‘deepest conviction that unless Britain and the United States are joined in a special relationship… another destructive war will come to pass’.

Increasingly, however, the relationship has come to be seen as one-sided, with British Prime Ministers more keen to flag up the alliance than US Presidents.

When David Cameron visited the White House in January, he insisted the President had said the special relationship was ‘stronger than it has ever been’.

The memo also expresses concern about a potential UK exit from the EU following an ‘Out’ vote in any referendum, saying: ‘Both the positive and the negative aspects of a prospective life outside the EU are more difficult to foresee.’