LUXEMBOURG (REUTERS) - Britain could not rejoin the European Union after leaving because the membership requirements would be too onerous, Foreign Secretary Philip Hammond said on Monday.
He said Thursday's referendum on EU membership offered "no way back".
Since becoming a member of the bloc in 1973, Britain has negotiated exclusions from EU laws and membership of the single currency, as well as winning a rebate from the EU's annual budget.
Such measures would not be available to a new member, Hammond said
"It simply wouldn't be credible for the UK to ever contemplate rejoining," he said.
"Any future British government wanting to join the EU would be faced with standard terms," Hammond said, citing membership of the euro zone, the EU's passport-free travel zone and areas of security and justice legislation.
"So this makes this a very asymmetric decision on Thursday. If we decide to go, we're gone, forever, no second thoughts, no turning back."
Hammond, a former businessman who once said he would vote to leave the EU if the bloc refused to change, backed staying in the European Union after Prime Minister David Cameron negotiated additional exemptions from EU policy in February.
Hammond, who became foreign minister in July 2014, also said there would be no need for debate in Britain's parliament if Britons voted to leave because the message would be definitive.
"It would be redundant, unnecessary and frankly insulting to the British people to have a debate in the parliament on something they have just spoken on in a referendum," he said.
However, in the event of a British exit, Hammond was less clear on when Britain would inform the other 27 European Union governments it was activating the formal exit clause in the EU's rule book.
The so-called Article 50 allows a member state to notify the European Union of its leaving and obliges the bloc to try to negotiate a "withdrawal agreement".
Hammond said it would have to be served "at some point" but it would be up to the government to decide.
"I hope the eventuality won't arise, because if we do serve an Article 50 notice, that is an irreversible process. It means we are out and there is no way back."