Former defence secretary Liam Fox has urged the Conservative Party not to use any of its money to back the campaign to stay in the European Union.
He said funds and other party resources must not be put behind either side of the referendum argument given the Tories were clearly split on the issue.
The MP rebelled earlier this week over plans to scrap "purdah" rules limiting government announcements before a vote.
David Cameron has said a referendum will be held before the end of 2017.
The issue of the UK's membership of the EU will be officially discussed by its 28 leaders for the first time on Friday when they gather for a European Council summit in Brussels.
Mr Fox told the BBC's Sunday Politics he would decide which campaign to back once the extent of the changes Mr Cameron was seeking became clearer and what the response of other EU leaders was.
But he said he was adamant that the Conservative Party, as a campaigning organisation, should remain neutral on the issue as it was not a "party political" matter.
"I think the Conservative Party itself is likely to be split in two ways," he told the BBC's Sunday Politics.
"I think the parliamentarians inside the Conservative Party will have a split in one direction. I'd have thought the party activists and membership might be very split on another.
"And I would have thought it would be those activists and members who would object to not just the party's money... but the party's machinery - our email addresses and the information we have on voters - [being used].
"That should not be used in my view to campaign in a party political way in a campaign which will not be party political."
'Friendly and democratic'
Asked if the prime minister would risk splitting the Conservative Party by insisting on collective responsibility from cabinet ministers, Mr Fox added: "If the prime minister was to insist on cabinet collective responsibility during the referendum I'm sure he could get it - I'm just not sure he would have the same cabinet."
Pressed on whether that meant some ministers would resign, he said he was "pretty sure" that would happen given this was the "only opportunity" that many of them had had in their lifetime to have a say on the matter.
"If you're under 58 in this country you've never had a chance to have a say on our constitutional relationship with Europe, and it's too important an issue to allow it to be subjected to any other considerations."
Mr Fox's comments came as it was reported that a group of business figures are putting together an apolitical group to push the case for leaving, with a former army general said to be in talks to act as its leader.
"This is too important to leave to politicians," businessman Arron Banks, a UKIP donor who is spearheading its efforts, told the Independent on Sunday. "They can endorse it and support it but they will not be involved in the campaign."
Former Home Office minister Damian Green, one of the most prominent pro-European voices in the Conservative Party, said the EU had survived crises in the past and it would be "peculiar" for the UK to "detach itself from a large body of friendly democratic countries" at a time of growing instability in eastern Europe and further afield.
"Of course the EU is not perfect, but the achievement from the collapse of the Soviet Union in 1990 for the next 20 years is an extraordinary piece of statecraft if you look at it historically," he told Radio 4's The World This Weekend.