A Tory backbench rebellion over the EU referendum is set to go ahead after leading Eurosceptic Sir Bill Cash dismissed the Government's assurances over so-called purdah rules that restrict its ability to campaign in the run up to the vote.
Sir Bill said the Government's promise to bring forward future proposals to ensure a fair referendum was "bizarre" because backbenchers have no way of knowing what they will be.
The row over the European Union Referendum Bill has erupted because the Government is attempting to suspend the rules of purdah before the vote, arguing that it would interfere with day to day government business which involves the EU.
But Sir Bill - who has tabled an amendment to reinstate purdah 28 days before the poll - and other Eurosceptics argue that suspending the rules would allow the Government and EU Commission to bombard voters with taxpayer funded propaganda before the vote.
Chancellor George Osborne promised the Government would "come forward with reassurances" to make sure "there isn't an unfair referendum". Europe Minister David Lidington has also written to MPs to try to head off the rebellion.
But Sir Bill dismissed the assurances, saying the issue raises a question of trust, and that suspending purdah would be "unfair on the voters" and a "retrograde step" for democracy.
MPs are now poised for a crunch vote on the issue later today.
During the committee stage of the Bill, Sir Bill said: "The problem that we've got here is that this situation that we're in now is not necessary and I speak more in sorrow than anger about this.
"And I have spoken to the Minister for Europe and we've had a good discussion as we always do and I was grateful for the fact that he did mention in his own letter that he was grateful for the constructive way in which these concerns were raised.
"But I have to say they haven't allayed those concerns."
He added: "I put it this way round - that ultimately it raises a question of trust and the problem that we have been faced with is that there are extremely sound reasons for the provisions in Section 125 (of the Political Parties, Elections and Referendums Act 2000, which guarantee purdah)."
Delivering his speech sitting down, having spent the previous four days in hospital, he went on: "It's not just about trust because we don't know what the outcome of these discussions and consultations we're being promised will be.
"What we do know is that doing a referendum in a manner which is unfair on the voters is a very, very retrograde step in the kind of democracy that we uphold."
Sir Bill also denied the situation amounted to a "Maastricht moment", when former Tory PM Sir John Major was hit by a crushing backbench rebellion over Europe shortly after winning a general election.
He said: "By the way, this has absolutely nothing to do with Maastricht or anything like that - that was a rebellion because we didn't have a referendum.
"This is merely to make sure that the voters get a fair choice, that must be one of our prime duties."
Sir Bill added: "They have an absolute right to know that the way in which the referendum is conducted will in no way be recanted or manipulated, no way whatsoever - either way of the argument either yes or no.
"But by taking this out and then asking us to consider on the basis of consultations yet to come seems to me to be quite bizarre."
Meanwhile, SNP foreign affairs spokesman Alex Salmond endorsed Sir Bill's amendments.
Speaking before the debate, but moving his own committee stage amendments, Mr Salmond said: "I hear from the Government they can't really function in a purdah period, that government won't be able to make representations to European Council, that it will be so impossible over the 28 day period.
"But that's what happens in each and every general election we have had. I didn't notice recently, in April and May, that the administration of this country ground to a halt - a lot of people thought it was better not having a fully activated government in the campaign period.
"If it can be done in each and every general election, then it can be done in this referendum campaign."
Meanwhile, Tory former Cabinet minister Liam Fox also warned that if voters feel the referendum is rigged because the Government has used taxpayers' money to support one side of the debate, the consequences would be "intense".
He said: "If people believe that they have been bounced or that the result is the consequence of a rigged process it will become extremely difficult for that to happen and the political consequences in my view will be quite intense."
Shadow Europe minister Pat McFadden said Labour did not oppose the Government's wish to suspend purdah, but called for clarity over what it intends to do or publish during the referendum period.
He said: "More clarity is needed than has currently been made available through ministerial statements.
"What form will expressing a view take and what form will it not take?
"The Government needs to provide more information and more clarity and more reassurance on this point."