Cabinet ministers are among private backers of a 110-strong group of Tory MPs preparing for the possibility of Britain voting to leave the European Union, one of its highest-profile members indicated as David Cameron faced mounting pressure to change the rules of the referendum.
Eurosceptic former environment secretary Owen Paterson said there "will be support" from senior figures for the growing Conservatives for Britain initiative which says it backs the Prime Minister's renegotiation but is drawing up plans for if he fails to secure sufficient change.
And he signalled it is ready to flex its muscle in the Commons if ministers press ahead with proposals to scrap the usual "purdah" restrictions governing the use of the Whitehall and Brussels machine in the "short campaign" in the run-up to a national vote.
As Mr Cameron prepares to meet more EU leaders this week to bolster support for his effort to recast the UK's relationship with the EU, party grandee Lord Heseltine urged MPs not to make the PM's job harder by agitating in public.
But Mr Paterson insisted the group is "beefing up" his chances of success by demonstrating the level of concern.
He urged the Government to avoid a "totally home-made" showdown over the campaign rules by reinstating the ban on public body activity in the final 28 days of the campaign and urged Labour - which first introduced the rule - to help inflict a Commons defeat if they do not.
As many as 80 backbench Tories are said to be ready to back a rebel amendment.
"It is unacceptable that there will be no limit to local government, national government or above all European government agencies spending money and sending information to citizens. That is going to skew the whole thing," he told BBC1's Andrew Marr Show.
"This is a heartfelt plea to the Government: if it is seen to be rigged, if the British people don't think it's fair, then whatever the result it won't be seen to be legitimate and this whole issue will fester further.
"If this whole issue is swamped up to polling day it will not be a fair poll, so I really would ask the Government very simply just to withdraw this whole thing and go back to the 28 days.
"We really don't want an argument over this; it is totally home-made."
Pointing out that senior Tories in opposition had argued for a longer period, he said: "I really do expect the Labour Party to go along with their own legislation."
Mr Paterson declined to reveal the names of any members of the group, but pushed on reports they include Cabinet ministers, he said: "There will be support, yes."
Mr Cameron was forced to backtrack last week on suggestions that Cabinet ministers who want to campaign for an exit would have to quit their jobs.
Lord Heseltine said the rebels should show more respect to Mr Cameron for having secured a Conservative majority against the odds in May's general election.
"Virtually without doubt, with the remarkable work of George Osborne, it is David Cameron who won that election for the Conservative Party and I would have hoped that, given that there is going to be a referendum, people would understand the complexity of what he's got to do and the prizes he might win and not confuse his negotiation with a sort of backdrop of campaigning which, in my experience, can only confuse the job he has to do and make it harder," he said.
The peer also played down the insistence by other EU states that the principle of free movement is inviolable, insisting it has "already been breached" by the use of transitional arrangements for new entrants.
"It could be seriously argued that, while accepting the principle, if the scale gets out of kilter for a particular reason like imbalance in the success of the economies then there has to be an agreed pace at which the process should go."
It is reported that Conservatives for Britain has the support of 110 Tory MPs, with 13 peers and 12 MEPs also wanting to join.
Mr Cameron is due to meet with the leaders of Italy, Luxembourg, Slovenia and Slovakia as he seeks to speak to all member states ahead of an EU summit at the end of the month, where Britain's demands will be discussed.
It is reported he will use that gathering to launch a second phase - a "technical study" by UK and EU officials of the sought reforms.
The Sunday Times also reports that the No camp has recruited a consistent critic of the PM - ex-Michael Gove aide Dominic Cummings - to advise on its setting up.
A Labour-led No campaign is also expected to be launched this week, backed by major party donor John Mills.
Meanwhile, shadow business secretary Chuka Umunna said Labour should not campaign alongside David Cameron or other Tories for a Yes vote to remain in the EU.
But he denied Labour's downfall in Scotland was due to it joining Tories to campaign for the maintenance of the union during the independence referendum, stressing the party also got "hammered" in the 2007 and 2011 Holyrood elections.
Mr Umunna told Sky News's Murnaghan programme: "What would be wrong is for us to conclude that just because we got involved with Better Together, which involved other groups and other political parties, that that brought about our defeat, because look we got hammered in 2011 and in 2007, long before that referendum came.
"We have got to be clear with David Cameron and others, look we're not going to be accepting a reformed European Union or a renegotiated settlement which involves a race to the bottom in people's terms, conditions and wages.
"But we've got to make an emotional argument for staying in.
"That will be the first stage of a wider argument, I think we do have to have a separate Labour Yes vehicle, but I also believe we need to be working with a variety of other groups in civic society, not just business, this cannot look like some cosy cartel of big business and established parties making this argument, a broad range of groups need to be making an emotional argument for Britain's membership of the European Union."