Nigel Farage has branded Conservative calls for the Brexit party to stand down in Labour marginals as “almost comical”, saying his party needs to get MPs into parliament to hold Boris Johnson’s feet to the fire.

The Brexit party leader said he still intended to stand candidates in about 300 seats held by Labour and pro-remain parties, having agreed to help the prime minister by withdrawing candidates in 317 seats won by the Tories in 2017.

However, he left the door open to holding back in other areas if Johnson made a further concession, such as standing down Conservatives in seats where they had no hope of winning.

“It’s almost comical. I’ve gifted the Conservative party nearly two dozen seats and I did it because I believe in leave. If they believed in leave, they would stand aside in Labour areas where the Conservative party hasn’t won in 100 years and will never win,” he told BBC Breakfast.

Farage claimed he made the decision not to stand candidates in 317 areas after watching Johnson’s video promising a Canada-plus trade deal with no political alignment with the EU by the end of next year.

Play Video 2:08 Nigel Farage reveals Brexit party will not stand in 317 Tory seats – video

He denied being motivated either by offers of a peerage, insisting he was “not for sale”, or by pressure from Donald Trump, who is friends with both Farage and Johnson and has called for an electoral alliance between the two parties.

“It’s a wild conspiracy theory, not true,” he said of Labour’s claims that Johnson and Farage had made a secret pact, with Trump “pulling the strings”.

Behind the scenes, Eurosceptic Tories have been urging Farage to give the Tories a clear run at Labour-held marginals, with some still convinced he could give more ground in the coming days if there was a further choreographed gesture from Johnson. Nomination papers have to be in by the end of Thursday.

Q&A What is a ‘marginal’ seat? Show Hide Marginal seats are parliamentary constituencies that have had a history of changing hands between parties, or in which the incumbent MP has a very small majority. Eleven seats were won by fewer than 100 votes in 2017. Often parties will target marginal seats with extra campaigning resources, as they are the places where they feel they are most likely to affect the balance in the House of Commons. There are several seats, including Kensington, Dudley North, Southampton Itchen and Newcastle-Under-Lyme, where the 2017 margin between the Conservatives or Labour winning the seat was between 20 and 30 votes. The most marginal seat in the country, though, is North East Fife, held by the SNP over the Liberal Democrats by just two votes. A three-way marginal, where the vote has recently been close between three parties, is much rarer.

In an email to supporters on Monday night, Farage said: “We will contest every seat held by Corbyn’s remainer Labour party, which has openly betrayed five million Labour leave voters. And we will stand in all seats held by the other remainer parties.”

The Brexit party claimed it was helping the Tories by standing candidates in Labour-held areas, as it was taking votes off Corbyn’s party more than the Tories in places such as the super-marginal seat of Bishop Auckland. However, it is more widely believed by pollsters that Farage takes about two votes off the Conservatives for every one vote from Labour.

Political experts were split over whether the decision would have much impact. Polls suggest the Tories would hold on to many of their seats but need to win key Labour targets to gain a majority.

Chris Curtis, YouGov’s political research manager, said: “Farage’s decision to stand aside in current Conservative-held seats and not in Labour-held seats that the Tories will be looking to gain will likely make very little difference.” He said it was “unlikely to be a game-changing moment”.

However, James Johnson, a former polling expert for Theresa May, said the substance of Farage’s move would be significant. “On paper, Farage’s decision still makes it harder for the Tories to gain seats. But in practice I expect this will seriously harm the general attractiveness of the Brexit party. Voters everywhere will see this as the party admitting that a vote for it is one that could undermine Brexit.”

In Scotland, the Brexit party would field candidates in “most if not all” seats, except the 13 held by Conservatives, Farage said. Asked about candidates and members who might be feeling disheartened at being asked to stand aside for the Tories, Farage said: “Making a big decision like this is difficult.”

He said all those who had put themselves forward as candidates had been aware they might be asked to stand down and “put country first”. He claimed that at a meeting where they were told this, they applauded.