BREXIT would end Britain’s special relationship with America and break up the United Kingdom, Nick Clegg has claimed in his starkest warning yet about the consequences of leaving the European Union.

In a hard-hitting speech at Princeton University in New Jersey, the former Deputy Prime Minister, a staunch Europhile, said leaving the Brussels bloc would leave Britain weaker, economically and politically, and would lead to an “unravelling” of its most important relationships.

“With our neighbours in Europe, with our cousins in America and even with our brothers and sisters within the United Kingdom if it pushes Scotland closer to independence.

“We will be left with no empire, no Union and no special relationship. We will never have been so alone; never so isolated, never so powerless,” Mr Clegg declared.

“It is in Britain’s vital national interest, just as it is in America’s, that we remain a member of the European Union and a leader in the world,” he insisted.

But Matthew Elliott of Vote Leave dismissed the intervention from the ex-Liberal Democrat leader, saying: “Nick Clegg lied about tuition fees to get himself into government six years ago. He promised a referendum but then backtracked on it. He used to warn we’d disappear off the map if we didn’t join the euro. Now he’s telling us it would be a disaster if we Vote Leave along with his backers at Goldman Sachs.

“Voters will be in no mood to be lectured by a politician with more broken promises than there are Lib Dem MPs left in Parliament. We should Vote Leave and take back control of the £350 million the EU takes from UK taxpayers each week,” he added.

Earlier this week, leading Outer Michael Gove, the Justice Secretary, argued that Brexit would be a "galvanising, liberating, empowering moment of patriotic renewal" while Vote Leave sources said it would also lead to the break-up of the EU as we knew it.

But the former Liberal Democrat leader’s warning echoed that earlier from Larry Summers, who served under President Bill Clinton as Treasury Secretary, and who said the UK’s influence with Washington would be diminished along with its standing in the world if it opted to leave the EU.

Ahead of Barack Obama’s visit to the UK on Friday when he is expected to endorse the campaign for keeping Britain in the EU, Mr Summers along with seven other former US Treasury chiefs expressed concern that Brexit would be a "risky bet".

He argued that Brexit would “reduce Britain's very positive influence as an ally of the US. The special relationship would translate much less into prosperity for both our countries and it 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."

Other developments in the in-out debate included:

*the City of London said Britain's financial services sector would be the worst hit if Britain voted to leave the EU with “serious consequences" for the Square Mile's role as an international financial centre, including job losses at major banks;

*Whitehall would struggle to negotiate new international trade arrangements if there were a Brexit vote, according to Dominic Cummings, Vote Leave’s campaign director, who told MPs he had little confidence in the calibre of Whitehall officials, who would have to negotiate any new deals;

*more than 200 entrepreneurs have thrown their weight behind the Remain campaign, claiming the economic shock of Brexit would be "hugely damaging" to the prospects of start-up businesses;

*ex-Bank of England governor Lord Mervyn King questioned the Treasury's analysis that leaving the EU could cost every UK household £4,300 a year, saying the issue could “not be reduced simply to the simple-minded level of a cost-benefit analysis" and

*a poll showed substantial majorities in four European countries wanting Britain to stay in the EU – Germany(78 per cent), Spain (67), France (59) and Poland (54).