Tory eurosceptics were given a boost when two cabinet ministers publicly declared they would vote to leave the European Union (EU) on the basis of the present relationship with Brussels.
Defence Secretary Philip Hammond joined Education Secretary Michael Gove in saying he would back a British exit if there was an immediate referendum.
Both senior Government figures insisted they backed David Cameron's bid to negotiate the return of some powers and put the resulting relationship to voters in an in/out referendum by 2017 if the Tories win the next general election.
But their comments will encourage backbenchers seeking to put pressure on the Prime Minister to bring forward legislation enshrining that pledge in law in a bid to win back voters from the UK Independence Party.
Mr Gove said he was prepared to contemplate cutting formal ties with the EU, suggesting he was in line with public opinion in feeling it would be "perfectly tolerable" and indeed bring some benefits.
Asked about reports last year that he told friends he would vote "no" if there was a referendum now, Mr Gove told BBC1's Andrew Marr Show: "Yes, I'm not happy with our position with the European Union. But my preference is for a change in Britain's relationship."
Mr Hammond later told Pienaar's Politics on BBC Radio 5 live: "If the choice is between a European Union written exactly as it is today and not being a part of that, then I have to say that I'm on the side of the argument that Michael Gove has put forward. I believe that we have to negotiate a better solution that works better for Britain if we are going to stay in."
Pressure for action from the Tory right has been fuelled in recent days by exit calls from several party grandees including ex-chancellor Lord Lawson. Mr Hammond criticised that position as "defeatist" and said he believed Mr Cameron had a "reasonable chance" of securing sufficient reforms.
Mr Hammond and Mr Gove, along with Home Secretary Theresa May, indicated that they would abstain in a expected Commons vote attacking the failure to include legislation paving the way for a referendum in the Queen's Speech.
Ministers have been excluded from a free vote granted to Tory backbenchers to back the rebel amendment - with around 100 tipped to take the highly-unusual step of formally criticising their own government's legislative programme.