Former Tory leader Lord Howard has joined calls for ministers to be allowed to campaign for Britain to leave the EU.
The peer said Cabinet members who "feel strongly" about the issue should be free to speak out ahead of the looming in-out referendum.
The comments will increase the pressure on the Prime Minister to grant his top team a free vote when he completes his drive to renegotiate the UK's membership terms.
Several Cabinet ministers are thought to be ready to quit if Mr Cameron tries to make them collectively back an "in" vote.
Lord Howard told BBC Radio 4's World this Weekend: "It's not of course a question for now.
"For now cabinet ministers are too busy running their departments and they shouldn't have time to campaign one way or another on the referendum.
"But when it comes to the campaign, if there are cabinet ministers who feel strongly that we should vote to leave the European Union, they should certainly be allowed to do so without losing their seat."
The intervention comes after former Conservative leader Sir John Major called for collective responsibility to be maintained throughout the referendum campaign.
Ex-deputy prime minister Lord Heseltine has warned that Mr Cameron would be a "laughing stock" if he granted his team a free vote.
Mr Cameron has said he is hoping to finalise a deal on new membership terms in February, with the nation expected to go to the polls in June.