Britain's relationship with the United States would be damaged if it chose to leave the European Union, according to former US Treasury secretary Larry Summers.
In a strong intervention in the Brexit debate ahead of President Obama's visit to London later this week, Summers said the UK would be "diminished" in world affairs if it voted to leave.
Summers is one of eight former US Treasury secretaries who have written in today's Times to warn Brexit would be a "risky bet" for the UK.
The intervention was welcomed by George Osborne who said it showed Brexit was "not a price worth paying".
Speaking to BBC Radio 4's Today programme this morning, Summers, who served in Bill Clinton's cabinet as well as Obama's National Economic Council director, said Brexit would "reduce Britain’s very positive influence as an ally of the United States".
"It would be unfortunate for the British economy, unfortunate for Europe, unfortunate for the United States, and unfortunate for the world. It would do damage to London as a financial centre, it would do damage to Britain as a gateway to Europe," he said.
"I believe that the pound would very likely come under very substantial pressure in ways that would ultimately lead to contraction in the British economy. So I do not think that words would speak loudly in the face of what would be perhaps the most isolationist deed in the last century for Britain."
He added: "I think the special relationship would have much less influence on the broad world. Much would be lost by the kind of split in the West that a British withdrawal from Europe would represent."
Summers said it was "striking" that both Democrat and Republican politicians, who agree on so little, agreed that he UK should remain in the EU.
Former Conservative defence secretary and Brexit campaigner Liam Fox told Today he had "respect" for Summers but said he and the other former US officials did not understand the problems with the EU.
"Unless there is fundamental change in that European leadership we will have an imploding continent. I think a British exit may give them the shock therapy they actually require to make the change necessary to stop Europe falling apart," he said.
Obama is expected to encourage British voters to choose to remain in the EU at the June 23 referendum during his visit to the UK.
The move has angered pro-Brexit campaigners, including former Conservative work and pensions secretary Iain Duncan Smith who has said the president is trying to "bully" British voters.