A change in British prime minister will not shift the fundamental realities of Brexit, Ireland’s deputy PM has warned, saying there is no chance of the EU ditching or watering down the Irish backstop.

Wholesale changes to Theresa May’s withdrawal agreement have been suggested by some as a way of avoiding a no-deal Brexit, but Coveney told BBC One’s The Andrew Marr Show on Sunday: “If the approach of the new British prime minister is that they’re going to tear up the withdrawal agreement, I think we’re in trouble. I think we’re all in trouble, quite frankly.

“That’s like saying, ‘Either give me what I want or I’m going to burn the house down for everybody’.”

A no-deal departure would not be the fault of the EU, Coveney said, but would be entirely down to UK political considerations. If it happened, he added, Ireland would need to impose some form of border checks with Northern Ireland to safeguard its position in the EU single market.

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 said: “Just because there’s a change in personality as British prime minister doesn’t mean that the negotiation of the last three years and the solutions that were designed by the British government as much as by the EU aren’t still as relevant and important today as they were six or eight weeks ago.”

While the Sunday Times has claimed Ireland and other EU nations could be willing to give way to a Johnson government to avoid no deal, Coveney said major changes to the backstop – the insurance policy to avoid a hard Irish border if no other solution can be found – such as adding a time limit, were simply impossible.

“We’ve always said no to that,” he said. “There’s a context around this. You have to be able to answer the question, if asked, of what happens at the end of that time limit. And if you can’t answer that question then it’s not a backstop at all.”

The backstop, Coveney said, was “about reassuring people in Northern Ireland that they are not going to go back to the friction and tensions of the past”.

He said: “To ask Ireland to compromise on that core issue, when we spent two and a half years working with the British government and the EU to try and find a way to compromise on all sides to ensure that we don’t face that prospect, it is not a reasonable ask, because of a political challenge in Westminster, to move away from that position.”

Coveney said he understood only too well what the consequences for Ireland and Northern Ireland would be of no deal, but said this would entirely be down to UK government choices.

“It is not where we are now. If the House of Commons chooses to facilitate a no-deal Brexit, and if a new British prime minister chooses to take Britain in that direction, then it will happen. But this will be a British choice. Not an Irish choice, not an EU choice – this is a British choice,” he said.

He said everyone wanted to avoid no deal, having worked for three years to do that. “I have done little else in politics apart from focus on the choice of the British people to leave the European Union and its impact on Ireland and the EU,” he added.

“We want to try to resolve these issues, but we won’t do it on the basis of being told what must happen because that is the only thing that will pass in the House of Commons.

“And international negotiation needs to involve compromise on both sides. We have a deal that has involved compromise on both sides, to facilitate, let’s not forget, British red lines throughout this process. And a new British prime minister doesn’t change that.”

In the event of no deal there would be some sort of checks on goods away from the border, Coveney said, to protect Ireland’s place in the EU single market. He added: “What I mean is, we cannot allow an open back door into the EU single market through Northern Ireland. If we do then the Republic of Ireland would be taken out of the single market as well, and I can’t allow that.”