David Cameron faced further pressure to respond to the rise of Ukip, with a move by Tory MPs to promise to vote for the UK to leave the European Union and calls from a grassroots organisation to take back immigration powers from Brussels.
The Prime Minister suffered a major blow after Douglas Carswell defected to Nigel Farage's party, with polling indicating he would comfortably win the by-election in the Clacton seat for Ukip.
Now dozens of Tory MPs are expected to promise to vote for the UK to leave the European Union in an effort to win support from Eurosceptic constituents at the 2015 general election.
The Independent reported that up to 100 Conservatives would pledge to vote to leave the EU, no matter what changes to the UK's terms of membership Cameron was able to achieve before the referendum he has promised by the end of 2017.
Mark Reckless, the Tory MP for Rochester and Strood, told the newspaper: "My election manifesto will confirm my view that we should leave the EU."
Other Eurosceptics predicted that between 50 and 100 Conservative MPs would make the same pledge in their election material.
Tory MP Jacob Rees-Mogg said he had not been approached but would be tempted by the idea unless Cameron set out the conditions under which he would campaign for an exit.
Rees-Mogg, who has called for the Conservatives to do a deal with Ukip at the election, told BBC Radio 4's Westminster Hour: "In terms of a personal pledge, I was very interested to hear that. I hadn't heard of it before, nobody has approached me to say would I do that.
"If the Prime Minister hasn't, by the election, set out very clearly his terms for renegotiation and and the circumstances under which he would call for a No vote it would be extremely tempting to put a personal statement in one's election address to say that I will vote for Out because I haven't seen what In will be."
The move came as the grassroots ConservativeHome website published a manifesto demanding a repatriation of power over immigration policy from the EU to let Britain "regain full control of its borders".
"With this renewed freedom, the existing immigration targets would be replaced by a points-based system emphasising the skills needed for economic growth," it said.
"New migrants would be required to purchase their own health and welfare insurance cover - and access to public services would have to be earned."
Eurosceptic Tory MP Chris Kelly announced he would stand down from his Dudley South seat at the next election after just one term, although he did not reveal the reasons behind his decision.
Conor Burns, another Eurosceptic MP, said he was "very disappointed" by the announcement and added: "We are losing the wrong people."
Ukip leader Nigel Farage said a by-election win for his party in Clacton would signal a "landslide in British politics".
A Survation poll for the Mail on Sunday gave Ukip a massive 44 point lead over David Cameron's party and Mr Farage predicted that a win would result in "more and more people coming over to us".
The figures prompted Tory MP Nigel Evans to tell Sky News: "There is a little bit of me that says perhaps we should sit this one out and let Douglas get on with it because it's a total distraction quite frankly."
But Defence Secretary Michael Fallon said Carswell's defection was "rather illogical" as only the Tories could deliver a referendum.
He told Sky News' Murnaghan programme: "The only prospect of delivering an in/out referendum is a clear Conservative majority at the next election.
"Anything else and you'll end up with a Miliband government that wants no change at all in Europe."
Dismissing Farage's claim that he could hold the balance of power, Fallon said: "We are after a full majority Conservative government at the next election and the reason we want that is that is the only way we can deal with this European issue by going for the reforms we need and then having a referendum and giving everybody a chance to vote.
"Only a Conservative government will do that. The Liberal Democrats aren't interested in it, Labour won't change anything in Europe and Ukip can't change anything in Europe."
He rejected the suggestion that there could be local pacts between Eurosceptic Tories and Ukip.