David Cameron has acknowledged for the first time that Britain could decide to leave the European Union, by saying that an exit is “imaginable.”

While insisting that he does not want the UK to withdraw from the EU, he appeared to raise the prospect that the British people might decide otherwise. Asked in the Commons if he could ever imagine Britain leaving, he replied: “All futures for Britain are imaginable.” He added: “We are in charge of own destiny, we can make our own choices. I believe the choice we should make is to stay in the EU, to be members of the single market, to maximise our impact in Europe, but where we are unhappy with parts of the relationship we shouldn't be frightened of standing up and saying so.“

He gave further comfort to hardline Conservative Eurosceptics by ruling out an “immediate ‘in or out’ referendum.” His use of the word “immediate” suggested that he might contemplate an eventual “in or out” vote, an option he has previously downplayed.

The Prime Minister was making a Commons statement on last week’s summit of EU leaders in Brussels, where he said he would seek “opportunities” for Britain to repatriate powers in return for allowing eurozone countries to forge ever-closer economic and monetary union.

Although aides insisted Mr Cameron was repeating his view that he did not support either withdrawal or the status quo, his language suggested he would announce a tougher line in a long-awaited speech on Europe next month in which he is expected to promise a referendum.

Labour seized on today’s remarks, with a senior party source saying: “It is clear David Cameron is heading towards an ‘in or out’ referendum. This will cause massive economic uncertainty and drive away inward investment.”

Douglas Alexander, the shadow Foreign Secretary, said: “David Cameron's long-anticipated speech seems to be as much about the [2014] European elections as about the EU. He seems to be being driven by external pressure from UKIP and internal pressure from his backbenchers towards an ‘in or out’ referendum in contradiction of what he himself was arguing only a few weeks ago.”