Boris Johnson has been publicly offered a non-aggression pact by Nigel Farage if he signs up fully to a no-deal departure from the EU and the Conservatives stand aside in more than 80 seats, amid signs the prime minister could be considering softer options for a deal with Brussels.

The Brexit party leader urged the prime minister to strike an agreement to stand jointly in favour of no deal, at a time of increasing speculation that Johnson is revisiting ways of making a version of Theresa May’s withdrawal agreement work.

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Johnson insists he is not proposing a Northern Ireland-only backstop, which EU leaders hope he could move towards, as that would not be acceptable to the Democratic Unionist party. However, the idea of an all-Ireland agrifood zone of regulatory alignment as part of the solution was discussed on Tuesday by Johnson and Arlene Foster, the DUP leader, as long as it did not extend to an all-Ireland customs area with a tariff border in the Irish Sea.

The idea would not be enough to satisfy Brussels that the backstop is no longer necessary, but the discussions were taken as a sign that Johnson is more focused than ever on finding solutions to the Irish border problem.

A No 10 spokesman said Johnson rejected the idea of an alliance with Farage, saying he “has been clear – no pact”.

It comes as the Sun reported that Johnson recently claimed to anti-no-deal Tory rebels that ultimately the hardline Eurosceptic Conservatives would be the ones putting “spears in my back” when they realise he is prepared to make a compromise on a deal.

The prime minister also told cabinet he was fundamentally liberal and should be regarded as a “Brexity Hezza”– a reference to the former deputy prime minister Michael Heseltine.

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Andrea Leadsom, the business secretary, did not deny Johnson was looking at softer options for a deal on Wednesday morning, as she appeared on BBC Radio 4’s Today programme. “We are looking at creative ways to ensure no Irish backstop,” she said. “We are looking at getting a good deal that works for the UK that is different to existing one.”

However, Farage is preparing to exploit any move by Johnson towards a return to May’s withdrawal deal, which the prime minister had promised his supporters was dead and that the backstop must be completely removed.

The Brexit party took out a full-page advert in the Sun and a wraparound ad in the Daily Express, setting out its terms for a hard Brexit alliance to take on tactical voting among supporters of the pro-referendum parties.

Farage would pledge to give the Tories a clear run at all their existing seats and targets if the Brexit party were to get a similar deal in 80-90 constituencies. Johnson would also have to sign up unequivocally to a no-deal departure.

Facebook Twitter Pinterest The Brexit party wraparound in the Daily Express.

He challenged Johnson: “If you have the courage for a clean-break Brexit, then we will help you secure a big Brexit majority. Together we can destroy Corbyn’s Labour”.

Farage’s high-profile adverts appear designed to publicise his harder Brexit position than Johnson in the months before a possible election, with little chance that the prime minister could accept such a high price for a pact.

However, most recent polling suggests the Conservatives would be short of a majority with Farage’s Brexit party eating into his vote and the possibility of pro-referendum parties – Labour, the Liberal Democrats, the SNP and others – potentially able to cobble together a majority for a remain alliance.