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David Cameron has urged those campaigning for Britain to quit the EU not to turn the issue of Europe into a “neverendum” after his Eurosceptic opponents hinted they might continue the fight even if they lose the June 23 vote.

Boris Johnson, the Conservative MP who hopes to succeed Mr Cameron as prime minister, suggested the British public may demand another say if the EU continued to integrate further.

But Tory grandee Lord Heseltine launched an attack on the Leave campaign’s figurehead and the man who once succeeded to his Henley parliamentary seat.

The former deputy prime minister, a long-time supporter of Mr Johnson, was highly critical of the former London mayor’s campaign, in the course of which he has drawn parallels between the EU and the Third Reich. Mr Johnson also claimed incorrectly that Brussels had banned bananas from being sold in bunches of “more than two or three”.

Lord Heseltine said he would be “very surprised” if Mr Johnson ever became prime minister after his “preposterous, obscene” remarks during the EU campaign. “I fear his judgment is going,” the peer told the BBC.

Mr Cameron insists the vote is a “once in a lifetime” decision but he faces the prospect that disillusioned Conservative MPs and activists could continue to agitate for Britain to leave the EU even if the country votes Remain.

Nigel Farage, UK Independence party leader, said a referendum result that produced a narrow vote to stay in the EU would amount to “unfinished business”.

By suggesting the matter will not be settled conclusively by the referendum, Mr Johnson was signalling to Eurosceptics that he could continue the battle as prime minister. When asked on Tuesday whether the referendum would permanently settle the question of Britain’s place in the EU, he said: “I think that the difficulty is that you cannot vote for the status quo.

“There is no way in this referendum you can vote just to remain in the EU as it currently is. It will continue to get more centralised and move ever closer towards a single federal political unit. That, I think, will be difficult for the British people.”

Mr Farage said: “If we were to lose narrowly, there would be a large section, particularly in the Conservative party, who would feel the prime minister is not playing fair.”

Leave campaigners have criticised the government for spending £9m of taxpayers’ money on a leaflet promoting EU membership and sending it to 27 million UK homes.

In the event of a narrow victory for Mr Cameron, the Leave campaign would claim there had not been a fair fight. Some Tories might then look to a future Eurosceptic leader, such as Mr Johnson or Michael Gove, to have another go.

The prime minister’s allies are hoping a conclusive victory by at least 10 points would help to settle the issue. Many Tory MPs from the 2010 and 2015 intakes are less ideological on Europe than some of their older colleagues and have insisted the party should accept the result.

But Mr Johnson’s decision to back the Leave camp is seen by his critics as an opportunist move to exploit the party’s deep-rooted Euroscepticism, particularly among grass-roots activists who elect the leader.

If Mr Johnson were to succeed Mr Cameron, who has promised to stand down as prime minister before 2020, he would be under pressure from those activists to return to the Europe question.

Mr Farage told the Mirror: “In a 52-48 referendum this would be unfinished business by a long way. If the Remain campaign win two-thirds to one-third, that ends it.”

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UK’s EU Referendum: How people would vote

For a more detailed summary of opinion polling visit the FT’s Brexit poll tracker page

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Few expect Mr Cameron to win by anything like such a margin and the prime minister told a business audience at Mansion House in the City of London: “You cannot have neverendums. You have referendums. When people start talking about a second referendum before you have even had the first, you are demonstrating you are losing the argument.”

The Scottish referendum in 2014 was also supposed to settle the issue “for a generation” but independence remains a live topic in spite of it being rejected by a 55-45 per cent margin.

The term “neverendum” was coined to describe the successive referendums on independence for Quebec.

Meanwhile Mr Cameron told the Mansion House event, organised by the World Economic Forum, that the only world leaders likely to rejoice in a Brexit vote were Vladimir Putin, the president of Russia, and Abu Bakr al-Baghdadi, leader of the Islamist extremist group Isis.

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