David Cameron will make himself an international "laughing stock" if he bows to demands to give ministers a free vote in the upcoming referendum on Britain's membership of the EU, Tory grandee Lord Heseltine has warned.
The Prime Minister has been coming under growing pressure to allow ministers to campaign on either side of the argument when the referendum - which must be held before the end of 2017 - is announced.
However, Lord Heseltine warned that a free vote for ministers would undermine the Prime Minister's authority and plunge the Conservative Party into a bitter civil war from which it would struggle to recover.
"I think that would be to make the Prime Minister a laughing stock across the world," he told the BBC Radio 4 Today programme.
"To have a civil war within the Conservative Party at that time in the belief that the referendum, having been determined, the participants in the civil war are going to sit round the table and happily smile together I think is rather naive.
"The consequence of having a free vote would be that the divisions, the divisiveness, the bitterness that would flow would actually, in my view, make the Prime Minister's position look very difficult."
The strongly pro-EU former deputy prime minister said ministers who were not prepared to campaign for reform of Britain's membership terms in a deal negotiated by Mr Cameron should quit the Government.
"If they feel so strongly, then they should resign - although it is quite difficult for me to understand how they are in the Cabinet in the first place if they feel so strongly," he said.
However Eurosceptic 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.
He said it would lack credibility to expect long-standing Eurosceptics in the Government to campaign for Britain to remain in the EU.
"Considering how incredibly thin the Prime Minister's demands are and what he is likely to be granted by the other 27 (member states), it will be wholly incredible for some figures in the Cabinet to campaign for that," he told the Today programme.
"We have a real responsibility to the British nation to come together again after the referendum to carry on governing properly. I think the way to do that is to let Cabinet ministers - once the settlement is seen to be what it is - to make a decision and to campaign for whichever side they want in the referendum."
His views were echoed by Graham Brady, the influential chairman of the Conservative backbench 1922 Committee.
"The fact that so many ministerial colleagues have spent decades campaigning on this, I don't think anybody could possibly believe it if they were all corralled into holding a line they clearly didn't all believe in," he told the Today programme.
"I think the collective responsibility should hold until it is clear what the deal is. It is entirely sensible that the Prime Minister would expect ministers to back the efforts to get a better deal in the EU. When we know what the shape of that is, we may need to go to another place."