Boris Johnson will tell the outgoing European commission president, Jean-Claude Juncker, on Monday that he will defy a new act of parliament and refuse to discuss or accept any offer to extend the UK’s membership even if a Brexit deal cannot be agreed, Downing Street said last night.

The prime minister’s hardline message to Brussels that he will take the UK out of the EU on 31 October, come what may, will enrage MPs from all parties who joined forces last week to force through a new law mandating him to ask the EU to prolong UK membership from the current deadline of 31 October until 31 January next year, to avoid a no-deal outcome.

Over a working lunch in Luxembourg – at which the EU’s chief negotiator, Michel Barnier, the UK Brexit secretary, Stephen Barclay, and Johnson’s Brexit adviser David Frost will also be present – Johnson will tell Juncker that the onus is on the EU as well as the UK to redouble efforts to find a deal. However, if those efforts fail he will be equally clear that he will not discuss or accept any extension and will simply take the UK out of the EU with no agreement.

The prime minister is expected to tell the commission president: “We’re leaving on 31 October, come what may – so let’s work hard to get a deal in the time remaining. Some MPs have been peddling a myth that I am not serious about getting a deal. Nothing could be further from the truth. I am striving for a deal and I think we can achieve this. I will commit UK officials and my lead negotiator to work flat out to come up with a new agreement without being trapped into EU laws.”

He is expected to add: “There should be no doubt about my determination to take us out on 31 October. I will not ask for an extension. I absolutely believe that our friends in Europe want an orderly exit, so now is the time for serious talks.”

A No 10 source said there was no chance of the prime minister going cap in hand to the EU summit in mid- October. “The PM will not negotiate a delay at the Brussels council,” the source said, before suggesting there could be legal challenges.

“We expect there to be a major court battle immediately after the 19th [of October] and attempts to pass legislation revoking article 50, which the prime minister will refuse to consider in any circumstances. Conservative MPs who want another delay don’t understand that this would probably destroy the Conservative party and put [Jeremy] Corbyn into No 10.”

Johnson took aim last night at opposition parties and the 21 rebel Tories who voted for the legislation to block no deal before being stripped of the party whip. In a statement, he told voters: “Don’t be fooled by Corbyn and the ringleaders. On the one hand, they say I don’t want a deal. On the other, they want to force me to extend. Both are wrong. I am straining to get a deal, but I will also end the uncertainty and take us out on 31 October.”

Keir Starmer, the shadow Brexit secretary, warned the prime minister not to pursue an illegal strategy. Photograph: Ollie Millington/Getty Images

As the row escalated, Keir Starmer, Labour’s Brexit spokesman, said the prime minister should think carefully before pursuing an illegal strategy. “If Boris Johnson seeks to defy the law, then the consequences will be serious,” he said. “Parliament has placed robust legal duties on Boris Johnson to secure an extension and stop a no-deal Brexit at the end of October. If the prime minister thinks parliament is going to stand by while he breaks the law, then he has another think coming.”

Johnson’s attempt to frame the Brexit argument as one between obstructive MPs in parliament and the 17.4 million British people who voted to leave the EU now threatens to spiral into a confrontation involving the courts.

Downing Street is, according to insiders, examining whether it can challenge in the courts the interpretation of the new legislation (the so-called Benn act) to block no deal and is looking for other ways to prevent Johnson having to seek an extension.

On Saturday David Davis, a former Brexit secretary, said he believed there might well be a way “to effectively legally kill off the Benn bill”.

Separately the supreme court will begin a joint hearing this week on whether the prime minister’s decision to suspend parliament for five weeks until mid-October was lawful, after legal challenges in the English, Scottish and Northern Irish courts.

While English and Northern Irish judges have ruled that the suspension of parliament was lawful, three Scottish appeal court judges concluded last week that the decision was unlawful, as suspension was clearly an attempt to “stymie” the House of Commons and prevent MPs from scrutinising the government over Brexit. A final supreme court ruling is expected this week or early next week.

Joanna Cherry QC, the SNP MP who jointly led the successful legal challenge in Scotland, said she was “cautiously optimistic” that the supreme court would follow the Scottish judges on the grounds that, while the courts cannot subject the executive to political scrutiny, they have a duty to ensure that the body charged with doing so – parliament – is able to perform that task.

Downing Street said that David Frost was making good progress in talks with DUP leaders, which could lead to a breakthrough on the issue of the Irish border. While the DUP leader, Arlene Foster, has denied that her party is shifting its position and would now be prepared to accept regulatory checks in the Irish Sea and some regulatory divergence from Britain – assuming there is consent from the provinces – Johnson’s officials are determined to show that they are serious about avoiding a no deal.

Speaking at a People’s Vote rally in Belfast on Saturday, the former Tory MP and attorney general Dominic Grieve, one of the 21 Conservatives who lost the whip, said Brexit would harm the Northern Irish economy whatever deal was reached and, in turn, a border would endanger peace. “Whether the rules in Northern Ireland follow those of the EU or those of the rest of the UK, one thing is certain – there will be an economic border placed in the way of business in Northern Ireland which does not exist today.”

In a sign of divisions in cabinet over Brexit on Saturday, culture secretary Nicky Morgan said that if there was a second referendum she would vote Remain. “My instincts are that I was sorry that the Remain campaign didn’t win in 2016 and that really I’m sorry that we’ve seen all the division and uncertainty over the last three-and-a-half years,” she told the BBC.

She added she voted to stay in the EU “for the same reasons that I felt very firmly back in 2016 and I campaigned for Remain” - which she said were both economic and geopolitical.