Boris Johnson has said the chances of a no-deal Brexit are a “million-to-one against”, despite promising to leave on 31 October whether or not he has managed to strike a new agreement with the European Union.

Johnson, the frontrunner to be prime minister, told a hustings that the chances of a no-deal Brexit were vanishingly small, as he believed there was a mood in the EU and among MPs to pass a new Brexit deal.

“It is absolutely vital that we prepare for a no-deal Brexit if we are going to get a deal,” he said. “But I don’t think that is where we are going to end up – I think it is a million-to-one against – but it is vital that we prepare.”

He said there was a new feeling of “common sense breaking out” among MPs in favour of passing a deal, despite many of his Eurosceptic backers believing he is readying himself for a no deal Brexit.

It comes just a day after he promised in a TalkRadio interview to leave the EU on 31 October “come what may, do or die”, raising fears among moderate Tory MPs and opposition parties that he was intending to push through a no-deal Brexit.

The EU has repeatedly said it will not revisit Theresa May’s withdrawal deal and experts are severely sceptical that a new prime minister can secure any changes to the controversial Northern Ireland backstop hated by Eurosceptics by the end of October.

Many of Johnson’s Eurosceptic backers are convinced that he will push through a no-deal Brexit by simply ignoring the will of parliament, where a cross-party group of MPs are planning to try everything possible to block this possibility.

However, Johnson was supremely confident that he could secure a new deal with the EU that would satisfy parliament. He played down the idea that he would simply sideline parliament or prorogue it in order to secure a departure on 31 October, but did not entirely rule it out. “I’m not attracted to archaic devices like proroguing,” he said.

The former London mayor also said it would be “absolutely crazy” to think about a general election before Brexit is done, despite the fact he could be forced into one if MPs vote down his government in a confidence vote.

He also said he would not give Nigel Farage a part in EU negotiations, saying the tactic is not to give other parties “the oxygen of publicity”. Challenged if that meant no role at all for the Brexit party leader, he replied: “It’s a pretty clear no ... absolutely not.”

Speaking before him at the same hustings, Jeremy Hunt, his opponent in the leadership race, took the opportunity to dismiss one of Johnson’s ideas that the UK could negotiate a standstill agreement to prevent trade tariffs with the EU if it left with no deal under an international mechanism called the General Agreement on Tariffs and Trade (Gatt 24).

“I think we’ve got to knock this Gatt 24 thing on the head,” he said. “You can only get an agreement not to introduce tariffs if both sides agree to that.

“So there isn’t a no-deal route that would allow us to take advantage of Gatt 24.”

Hunt said he would not accuse Johnson of lying but said that his proposals were factually not possible.

Hunt also ruled out an election before Brexit is sorted out and described Labour leader Jeremy Corbyn as a crocodile lurking beneath the water.

Earlier in a BBC interview, Hunt described 31 October as a “fake deadline” for leaving the EU in a move that drew derision from Tory Eurosceptics. His allies said afterwards that he had meant to say Johnson’s pledge to take the UK out of the EU by that date was a “fake promise” as it might not be deliverable. Johnson has repeatedly challenged Hunt to commit to the same deadline but Hunt has resisted on the grounds that it would not be sensible to rule out staying a bit longer if the UK were on the brink of a deal.

Q&A What does a no-deal or WTO-rules Brexit mean? Show 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

When quizzed on domestic policy, Hunt made a suggestion that the government should find a way to allow online voting in elections, saying it must be possible without it causing fraud.

The hustings involved pre-vetted questions from the general public being read out to the candidates by a moderator one after the other. Hunt has been pushing for Johnson to face him in head-to-head television debates before ballot papers are sent out to Tory members. However, Johnson has so far only agreed to one ITV event after this has happened, meaning many members will have cast their votes already by that point.

In a letter to broadcasters, Hunt urged them to hold the debates anyway with an empty podium for Johnson, in a move that would then allow him to present his arguments.

“I urge all broadcasters to press ahead with plans to hold live television events to scrutinise the candidates next week before ballot papers arrive, with or without Boris Johnson,” he wrote.

“You will make your own judgements about how best to format the programmes. He may still prove unwilling to subject himself to this scrutiny, but I do not believe it is fair or appropriate to deprive the other candidate of the chance to make their case on account of his shyness.”