The UK's future in the European Union (EU) is no longer a settled issue and it is not just "mavericks" calling for ties to Brussels to be cut, it has been claimed in the Commons.
David Cameron faced pressure from his own MPs to give a firm promise to hold a referendum on Europe, with calls for a simple in/out question.
The comments came as a backbench Tory bid to pass legislation which would allow the UK to leave the EU failed after running out of time.
Tory Douglas Carswell said a recent poll found a majority of voters want Britain to quit the EU, with 51% of people in favour of leaving and only 34% wanting to remain in. "This is the highest level of discontent for a generation," Mr Carswell said.
Britain joined the Common Market, Mr Carswell said, in the belief that it would be good for the economy. But the growth of countries such as China, India and Brazil, and the crisis in the eurozone, meant "far from joining a rising economic powerhouse, we have shackled ourselves to a corpse".
Mr Carswell, whose effort to repeal the 1972 European Communities Act failed to get a second reading after little over 30 minutes was available for the debate, hit out at the cost of EU membership.
"The 2010 increase in our net contribution is greater than the sum total of all the austerity savings made since the last general election," he said. "We are having to justify austerity in our constituencies when we have an administration that is handing over ever larger sums of our money to remain part of this austerity club."
Calling for serious discussions about how the UK could exit the EU, he said: "Withdrawing from the European Union can no longer be dismissed as unthinkable, it's no longer a marginal view confined to mavericks. It is a legitimate point that is starting to go mainstream."
Tory Edward Leigh said there was a "democratic deficit" as there had been no public vote on membership of the European club since 1975. He said a referendum would have to offer the choice of remaining in the EU or leaving in favour of a looser "customs union" with the continent.
Europe Minister David Lidington said the UK's membership of the EU was based on a "pragmatic" view of what was in the national interest. Mr Lidington added that EU membership was based on a "hard-headed, calculated and pragmatic decision" by governments including those led by "successive leaders of the Conservative Party".
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