David Cameron is facing mounting pressure to allow eurosceptic ministers to campaign for Britain to leave the EU after Nigel Lawson became the latest senior figure to call for members of the Government to be given a free vote.
The former Chancellor said letting ministers back the out campaign was "sensible" and would help the party unite once the referendum on the UK's relationship with Brussels had been staged.
It comes after former environment secretary Owen Paterson joined fellow ex-Cabinet minister Liam Fox in warning that giving ministers a free vote was the only way to keep the Conservative Party together.
But Tory grandee Lord Heseltine warned that Mr Cameron will make himself an international "laughing stock" if he bows to demands to let ministers speak freely.
Lord Lawson called on Mr Cameron to follow Labour premier Harold Wilson's decision to suspend Cabinet collective responsibility during the EU referendum of 1975.
He told the Daily Mail: "It would clearly be sensible in terms of party management to allow Cabinet ministers to speak out on both sides – not least to enable the Conservative Party to reunite easily once the referendum is over, whatever the result.
"That is what Wilson decided in 1975 and he was right. He may have been a bad prime minister, but he knew about party management."
Meanwhile, one of the party's most vocal eurosceptics claimed the Tory chairman had privately admitted "directing" Conservative donors into handing over money to the pro-EU campaign.
Steve Baker, chairman of Conservatives for Britain, said Lord Feldman had urged supporters to give money to the Britain Stronger in Europe campaign.
In an article for The Daily Telegraph, he wrote: "Lord Feldman, our chairman, has confirmed to me that he has directed donors to give money to the pro- EU BSE campaign in a personal capacity and in his spare time.
"He also tells me that he will point people in the direction of the Vote Leave campaign, but they are yet to receive a referral from our party chairman.
"Lord Feldman is a member of the political cabinet. His fundraising for BSE undermines the Government's policy of seeking real reform before deciding how to vote.
"Now he has admitted this new part time role, shouldn't the Prime Minster allow other cabinet members to help Vote Leave in their spare time?"