Michael Heseltine has warned the Conservative leadership candidates that they will never win a general election by following the “poisonous politics of Nigel Farage”, as he said the prospect of 100,000 Tory members choosing the next prime minister filled him with dread.

The former deputy prime minister, who was suspended from the Conservatives for voting Liberal Democrat in last week’s European elections, said the party would permanently lose his support and that of millions of other Conservatives by becoming more “Faragiste”.

Heseltine made the remarks in a speech to mark his appointment as president of the European Movement.

In a warning to the Conservative leadership candidates, he said: “Turn yourselves into branch offices of Brexit if you wish. But if you do so, you are on your own. Those upon whom you depend to win power in a general election will not come back. Good luck. Goodbye.

“Indeed, the prospect of a new prime minister being chosen by perhaps little more than 100,000 Conservative party members in the current circumstances fills me with dread. There will be an arms race in which candidates vie against each other for who can be the most Faragiste.”

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He said there was a danger that a new prime minister in favour of a hard Brexit could simply run down the clock to a default departure on no-deal terms. “Such a decision, which would deny either parliament or the people a say on a no deal outcome that neither wants, would be nothing short of a democratic and constitutional outrage.

“If successful, the consequences for businesses, for young people and for the integrity of the United Kingdom itself would rightly be hung around the neck of the Conservative party for a generation to come,” he said.

He went on to appeal to “every sensible Conservative MP, to potential leadership candidates, even to the Labour leader, not to force Brexit upon us now”, as he made the case for a second referendum.

Heseltine’s speech comes ahead of an intervention by Gordon Brown, the former Labour prime minister, warning against the politics of Farage.

Speaking in Peterborough, where Labour is fighting to keep its parliamentary seat in a byelection against strong support for the Brexit party, Brown will accuse Farage of “attempting to hijack British patriotism and representing a toxic, divisive, intolerant ‘them-versus-us’ nationalism more in tune with Le Pen and Putin than the values of the British people”.

He will also lambast Farage for taking donations under £500 through PayPal that could have been made from foreign and untraceable sources – something that is strongly denied by the Brexit party.

Brown will say: “He now says that his party has been financed by 100,000 British subscribers who have each given at least £25 – but his claim does not stack up. So the question remains: exactly where has the Farage money come from, and where does it come from now?”

The Electoral Commission attended the offices of the Brexit party last week to “review its systems” after Brown urged them to investigate concerns over the legality of the party’s funding. After the visit, a spokesman for the commission said: “Our review of the systems in operation by the Brexit party is ongoing.”

Farage has accused Brown of an “absolutely disgusting smear” against his party. “This from the man who was part of a Labour party who, through Lord Levy, were making a lot of big donors members of the House of Lords,” Farage said on a campaign visit to Exeter. He also said that “after seven and a half hours” the Commission officials had not “found a single misdeed”.

Richard Tice, the Brexit party’s chairman and co-founder, also insisted on Twitter that the allegations of illicit foreign funding via PayPal were unfounded: “The Brexit party only receives money in sterling. The offer stands to send a BBC journalist to come and look at our PayPal account.”