A group of former US treasury secretaries has warned that leaving the EU would be a "risky bet" for the UK.
George Schultz, Larry Summers and five other ex-White House advisers have backed the Remain campaign, saying it was better for Britain's future.
In an open letter in The Times, they called the referendum a "consequential decision" and said the special relationship benefited from having the UK in the EU.
"A strong Britain, inside the European Union, remains the best hope in our view for securing Britain's future, creating a more prosperous Europe and protecting a healthy and resilient global economy," they said.
The UK's ability to be a global centre of finance could also be at risk if the public voted for Brexit, they warned.
"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," the letter says.
The group of former treasury secretaries - which includes those who have served under both Democratic and Republican presidents - said that while trade deals would eventually be renegotiated, Brexit would "likely disrupt and reduce trade flows at least for a while, curtailing the scale and efficiency benefits from economic co-operation and integration".
They added that thanks to the special relationship, during difficult times, they had looked to Britain as "strong voice and partner" and claimed that "our collective efforts benefited from having a strong Britain within Europe".
The intervention was criticised by Brexit campaigners.
A Vote Leave spokesman told the BBC: "Not content with doing down Britain's economy, No 10 are now soliciting help from across the pond."