A British vote to leave the European Union would be "irreversible" and the country could only rejoin the bloc on what would be unacceptable terms, Foreign Secretary Philip Hammond warned Monday.

A British vote to leave the European Union would be "irreversible" and the country could only rejoin the bloc on what would be unacceptable terms, Foreign Secretary Philip Hammond warned Monday.

"The message we are trying to get across to the British people is that this is an irreversible decision --  if they decide to leave, there will be no going back," Hammond said as he arrived for a meeting with his EU counterparts in Luxembourg.

He said any idea that a vote Thursday in favor of leaving the EU could be followed by fresh negotiations with Brussels on a new deal was an illusion.

"Britain could never rejoin the EU at a later date except on terms that would be unacceptable -- membership of the euro, membership of (the) Schengen (passport-free zone) and so on," he said.

The most recent opinion polls show voters once more swinging behind the remain camp after a strong showing by Brexit supporters in the previous two weeks which rattled the markets badly and sparked a series of dire warnings about the consequences of Britain becoming the first country to leave the bloc.

Hammond said "the referendum battle ... is close fought. It will be a very tight result either way."

French Foreign Minister Jean-Marc Ayrault said British Prime Minister David Cameron was living up to his pledge to give the country a vote on the EU and while no one wanted to interfere, everyone in Europe was watching closely.

"All Europeans are turned towards the British people because our history reminds us of what we have lived through together, good and bad. The choice the British people will make will be important for them and for Europe alike," Ayrault said.