Eight former secretaries to the US Treasury have backed the "Remain" campaign ahead of the referendum on Britain's membership of the European Union.

In a letter addressed to The Times, the group warned that London's position as Europe's financial capital would come under threat in the event of a Brexit, while Britain's economic future as a whole could be in danger.

"We also find compelling the argument of the Remain camp that a vote to leave Europe represents a risky bet on the country's economic future," the letter said.

"Brexit could call into question London's role as a global financial centre. For many financial institutions, London has served as the financial springboard into Europe.

"EU membership allows banks based in London to sell their services across Europe without needing multiple regulatory approvals in each country. While Britain will remain an attractive centre for finance even if Britain exits, it should not take for granted its global primacy when it is no longer the gateway to Europe."

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In the letter, the eight former secretaries added the US were eager for the UK to remain part of the 28-country bloc from a political point of view as well as from a financial perspective.

"During our times as US treasury secretary, when confronting difficult global challenges, we often looked to Britain as a strong voice and partner," they said.

"We often found that our collective efforts benefited from having a strong Britain within Europe."

Should Britain leave the EU, it would have to renegotiate a number of trade agreements, which economists have warned could prove to be a very lengthy process.

"Over time, Britain would no doubt be able to re-establish ties through new trade agreements," the group said.

"But as our own experience in the United States with trade negotiations shows, it is a difficult environment to negotiate and approve agreements and the risk of accidents is real."

The letter comes a day before the US President Barack Obama, who has already offered his support to the pro-European campaign, is due to arrive in the UK for a state visit.

Obama is expected to reiterate his feelings about Britain remaining in the EU ahead of the 23 June referendum. However, the Mayor of London, Boris Johnson, one of the most high-profile figures in support of the "Leave" campaign, has urged the US president not to give his opinion on the subject.

The latest online opinion poll from ICM, of more than 2,000 people between 15 and 17 April, put "leave" one point ahead of "remain" (44% versus 43%), with 13% of respondents undecided.

The signatories of the letter are: George P. Shultz (1972-74), W. Michael Blumenthal (1977-79), Robert E. Rubin (1995-99), Lawrence H. Summers (1999-2001), Paul H. O'Neill (2001-02), John W. Snow (2003-06), Henry M. Paulson Jr. (2006-09) and Timothy F. Geithner (2009-13).