The Tories were thrown into fresh turmoil over Europe today as rebel backbenchers pushed for an early referendum on British membership.
Adam Afriyie MP said voters did not trust David Cameron to stick to his pledge to give voters a say on Europe after the 2015 election.
He said he would force a Commons vote on the issue in five weeks' time by tabling an amendment to fellow Tory James Wharton's private member's bill calling for an in-out referendum in 2017.
The move threatens to split the Tories and drew a furious response from Downing Street.
A No 10 spokesman said: "We will not allow this amendment to be passed under any circumstances. The PM will not let it stand."
Self-made millionaire Afriyie, who was linked to a leadership bid earlier this year, said he was not "convinced" Mr Cameron would deliver on his referendum pledge.
"Anyone under the age of 56 hasn't had their say on our membership of the European Union.
"What I am saying is we should have that referendum in 2014 and over 50% of the population want that, 80% want the referendum, 80% of businesses as well want the certainty, so I think this is a good thing.
"And, as a backbencher, I've got to examine my conscience.
"I couldn't sit here quietly and not give Parliament, every MP to search their soul and decide do you they really want to have this referendum?" he said.
Mr Afriyie denied he was manoeuvring for the top job but accepted his move could make him unpopular.
"I'm not going to be popular in the short term. I think people will come round over the next few weeks and realise somebody has got to give this option to the British people," he told Radio Five Live.
The Labour MP Tom Watson said he might back the amendment.
"I don't want to add to the PM's panic but I will probably be supporting Adam Afriyie with his amendment," he told BBC1's Andrew Marr Show.
"There are a lot of people on both sides of the House who think we need clarity on this now.
"The country has asked for it for a long time. Business is saying there is a lot of uncertainty."
But many Tory Eurosceptics rushed to distance themselves from the rebel MP.
Former Cabinet Minister Liam Fox said any attempt to amend Mr Wharton's Bill would be "a gift" to opponents.
The Tories "must unite," he said.
Mr Wharton warned the move could even "kill" his bill altogether.
"My concern is that any amendment, no matter how well-meaning it might be, is going to make the progress of the bill more difficult and it'll make it easier for those MPs who want to use procedural techniques to slow it down and stop it.
"I don't want to see that. I'd like to see my bill go through and I think this harms the chances of that happening," he said.
UKIP leader Nigel Farage said he would be "absolutely delighted" if Afriyie's gambit succeeded.
He said: "The Conservative problem is that what Adam Afriyie has done is put his finger on the real problem, and that is four years ago Mr Cameron gave us a cast-iron guarantee of a referendum, this time last year he was saying no referendum, he's now saying there should be a referendum, and people aren't quite sure what to believe."
Labour said it would make a decision on how to vote when it saw the text of the amendment.
Labour vice chair Michael Dugher MP said: "While millions of people worry about the cost of living crisis facing them right now, the Tories are back to obsessing about the European Union.
"We need a prime minister and a government that will make dealing with the cost of living the number one priority.
"Instead David Cameron is too weak and out of touch to stop this latest outbreak of Tory infighting."