Foreign Affairs Select Committee chairman Crispin Blunt said Prime Minister must not restrict ministers from campaigning for Brexit, despite Cameron's personal belief that the UK should stay in the European Union.
Speaking to Total Politics, Blunt said:
I think David Cameron would have lost his marbles if he tries to whip his administration to support his renegotiation position. I think it would be politically very difficult. This is a subject people feel very deeply about, it is a once in a generation time to settle this issue.
If we are in [the EU] we are not going to be constantly moaning about it and think we might be able to alter this.
The Prime Minister had previously said he was undecided on whether he would allow ministers to campaign with their views, after it was rumoured that he would sack ministers who campaigned for leaving the EU.
Cameron is hoping to secure favourable terms around reforming the UK's membership of the EU ahead of a referendum, which could be as early as next year and as late as 2017.
But he has had to concede on two key areas, having agreed to accept official purdah rules in the run up to the vote and agreeing to change the wording on the ballot, with the Electoral Commission suggesting the version put forward by government was biased.