Boris Johnson, who has said he would “rather be dead in a ditch” than delay Brexit, will face a legal obligation to do just that later on Monday and request an extension from the European Union.

How will that work with the EU?

The departing government asks for an extension, and EU leaders discuss (without the exiting state) and must reach a unanimous decision to grant more time. Despite the simplicity of the process, a few myths have piled up. One is that parliament’s no-deal bill could be treated by the EU as an extension request, regardless of whether the government sends a letter to Brussels. EU sources have described this interpretation as “complete nonsense”. Another myth is that Brussels could force an extension on the UK. This is also incorrect; the British government must agree to any proposed extension.

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

What does the EU think about delaying Brexit?

The UK has already had two Brexit extensions and the EU does not relish a third. EU diplomats say an extension would be granted for “a political event”, meaning an election or another referendum. An extension for other reasons is less certain. The UK would be expected to make a “reasoned request” showing the government could find a way out of the current Brexit morass. EU leaders do not want delay for the sake of a few months’ more political chaos at Westminster and “farcical” talks in Brussels.

Would France veto an extension?

France’s exasperation with Brexit was captured by its foreign minister, Jean-Yves Le Drian, who has said an extension beyond 31 October would not be possible under current conditions. Since April, more EU member states have come round to France’s point of view. The Netherlands, another close UK ally, sounds equally impatient. The Dutch trade minister, Sigrid Kaag, has suggested the certainty of Brexit on 31 October could be preferable to the current drift. “A good reason is needed for a new delay,” she said, using tough language that left little room for another extension.

It is hard to see the EU turning down a request for an extension drawn up by MPs intent on avoiding no deal. The EU has never wanted to be seen as pushing the UK out of the door. The German chancellor, Angela Merkel, who once described the UK as a “patient” that needed careful treatment, seems particular conscious of the weight of history. Meanwhile Ireland, an influential voice in Brexit talks, sounds open to an extension, to avoid the calamity of a hard border.

Couldn’t the UK get Hungary or Poland to block an extension?

This Brexiter idea is becoming almost as popular as the theory that German carmakers would demand a good deal for the UK. It is built on a similar misunderstanding of EU interests.

Neither Poland nor Hungary have anything to gain from frustrating the other 26 by vetoing an extension for Boris Johnson. Both are net recipients of EU funds and have their own fights with Brussels.

These countries’ reluctance to spend political capital on the UK was demonstrated at one EU summit. In front of the cameras, Hungary’s prime minister, Viktor Orbán, kissed Theresa May’s hand and told journalists he was opposed to “punishing the British”. Inside the privacy of the summit chamber, when he had the chance to speak for the UK, he said nothing.

How long could an extension last?

The current EU mood favours a short extension, with some diplomats reported to support MPs’ proposed 31 January 2020 date. But it is difficult to tell until EU leaders gather round the summit table. Before previous Brexit summits, delays of 12 and 21 months were mooted by senior officials, only to wilt under the scrutiny of EU leaders who take the decisions.

One wildcard is that the current EU leaders, Jean-Claude Juncker and Donald Tusk, are heading for the exits. Both have argued for longer extensions to allow the British time to resolve the crisis and prevent the EU from being bogged down by Brexit. Ursula von der Leyen, who takes over from Juncker as head of the European commission, is likely to be at the October summit. She has signalled she is open to an extension for “good reasons”.

Would the EU attach conditions?

Any extension would come with a familiar set of terms and conditions. The UK would have to pledge “sincere cooperation”, meaning it would not disrupt EU business. The previous extension agreement also included an often-forgotten clause that there would be “no reopening of the withdrawal agreement”.