Boris Johnson does speak to Steve Bannon, Nigel Farage has said, increasing the pressure on the Conservative leadership frontrunner to explain his links to Donald Trump’s controversial former campaign manager.

“Steve likes to be seen at the centre of the action. He knows Boris, he speaks to Boris. Steve speaks to virtually everybody,” Farage told a press conference when asked what he knew of Johnson’s links to Bannon.

The Brexit party leader said: “I’ve known Steve Bannon since 2012. There’s no great secret about that. Boris, of course, got to know Bannon when he was foreign secretary, when he was visiting Washington and going into the West Wing, and that’s how those two got to know each other.”

Farage’s comments follow the emergence of video footage in which Bannon speaks about his relationship and contacts with Johnson, and says he helped him put together his first speech after his resignation as foreign secretary, in which Johnson condemned Theresa May’s Brexit strategy.

Play Video 2:59 Steve Bannon says he advised Boris Johnson on key Leave speech linked to leadership bid - video

The footage, reported by the Observer, was shot in July last year by Alison Klayman, an American film-maker who followed Bannon over many months for a documentary called The Brink. The footage was not used in the film.

One clip shows Bannon reading a story about Johnson’s speech, before he says: “I’ve been talking to him all weekend about this speech. We went back and forth over the text.”

When reports emerged last year of links between Johnson and the far-right activist, who has tried to build up a network of populist movements across Europe, Johnson called them “a lefty delusion whose spores continue to breed in the Twittersphere”.

A spokesman for Johnson said of the footage: “Any suggestion that Boris is colluding with or taking advice from Mr Bannon or Nigel Farage is totally preposterous to the point of conspiracy.”

At the same event on Monday, Farage said he would back Johnson as prime minister, but only if he pushed for a no-deal Brexit on 31 October, saying he did not trust him to do this.

Q&A What does a no-deal or WTO-rules Brexit mean? Show Hide If the UK leaves the EU without a deal it would by default, become a “third country”, with no overarching post-Brexit plan in place and no transition period. The UK would no longer be paying into the EU budget, nor would it hand over the £39bn divorce payment. The UK would drop out of countless arrangements, pacts and treaties, covering everything from tariffs to the movement of people, foodstuffs, other goods and data, to numerous specific deals on things such as aviation, and policing and security. Without an overall withdrawal agreement each element would need to be agreed. In the immediate aftermath, without a deal the UK would trade with the EU on the default terms of the World Trade Organization (WTO), including tariffs on agricultural goods. The UK government has already indicated that it will set low or no tariffs on goods coming into the country. This would lower the price of imports – making it harder for British manufacturers to compete with foreign goods. If the UK sets the tariffs to zero on goods coming in from the EU, under WTO “most favoured nation” rules it must also offer the same zero tariffs to other countries.

WTO rules only cover goods – they do not apply to financial services, a significant part of the UK’s economy. Trading under WTO rules will also require border checks, which could cause delays at ports, and a severe challenge to the peace process in Ireland without alternative arrangements in place to avoid a hard border.

Some no-deal supporters have claimed that the UK can use article XXIV of the General Agreement on Tariffs and Trade (Gatt) to force the EU to accept a period of up to 10 years where there are no tariffs while a free trade agreement is negotiated. However, the UK cannot invoke article XXIV unilaterally – the EU would have to agree to it. In previous cases where the article has been used, the two sides had a deal in place, and it has never been used to replicate something of the scale and complexity of the EU and the UK’s trading relationship. The director general of the WTO, Roberto Azevêdo, has told Prospect magazine that “in simple factual terms in this scenario, you could expect to see the application of tariffs between the UK and EU where currently there are none”. Until some agreements are in place, a no-deal scenario will place extra overheads on UK businesses – eg the current government advice is that all drivers, including lorries and commercial vehicles, will require extra documentation to be able to drive in Europeif there is no deal. Those arguing for a “managed” no deal envisage that a range of smaller, sector-by-sector, bilateral agreements could be quickly put into place as mutual self-interest between the UK and EU to avoid introducing or to rapidly remove this kind of bureaucracy. Martin Belam

“He’ll have my support if he does the right thing. But I don’t know what he’s going to do,” Farage said. “I’d love to know who the real Boris Johnson is, I’d love to know what he really, really, truly believes. Our attitude, as the Brexit party, is why would we trust anybody?”

Speaking earlier to Talk Radio, Farage said he would consider a coalition with the Conservative party to ensure the UK left the EU without a deal.

“I’d do a deal with the devil to get a proper, clean Brexit,” he said.

Asked about the prospect of an electoral pact, Farage said: “If Brexit is not delivered, there might be some local deals done here and there, but if the Conservative party drop the ball on this then they’re toast.”

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He pointedly praised Johnson, claiming many Eurosceptics had joined the Conservative party in the past few months to ensure he became prime minister. Farage said Johnson would be hailed as “an all-time national hero” if he managed to face down parliament by delivering a no-deal Brexit by 31 October.

He also backed Johnson after a row with his partner, Carrie Symonds, on Thursday night was recorded by her neighbours and reported to the police.

“I think for people to record through the wall what is being said is absolutely disgusting,” Farage said. “There has to be a limit to this intrusion on people’s private lives.”

But he did say the frontrunner for the Conservative leadership should have answered questions about the row at a hustings event on Saturday. “All he had to say is that there was an argument and the police were called by malicious neighbours. Who hasn’t had an argument? That’s all he had to say,” Farage said.