In a defensive final speech of her premiership, Theresa May has taken a coded swipe at the “absolutism” of politicians like Boris Johnson as the Commons prepared for another tense showdown over efforts to halt a no-deal Brexit.

May took aim at the folly of those in her party who pursued ideological purity at any price, though repeatedly declined the opportunity for self-reflection when asked if she too had been willing to compromise only after being defeated.

“An inability to combine principles with pragmatism and make a compromise when required seems to have driven our whole political discourse down the wrong path,” she told an audience at the Chatham House thinktank.

“It has led to what is, in effect, a form of absolutism. One which believes that if you simply assert your view loud enough and long enough you will get your way in the end.”

May’s chancellor, Philip Hammond, who is expected to join the prime minister on the backbenches if Johnson is confirmed as the new Tory leader, also stepped up his attacks, calling it “terrifying” that one of Johnson’s close allies, Jacob Rees-Mogg, believed a no-deal Brexit will boost the economy.

The chancellor expressed his horror after Rees-Mogg used a Daily Telegraph opinion piece to dismiss the “pure silliness” of Treasury forecasts suggesting a £90bn hit to the economy.

Rees-Mogg claimed there were economic models that showed “the total positive impact of no deal could be in the region of about £80bn”.

Hammond hit back at the argument, saying on Twitter: “Happy to debate scale of negative impact of no deal on the economy – but terrifying that someone this close to a potential future government can think we’d actually be better off by adding barriers to access to our largest market.”

Rees-Mogg told the Guardian this was part of “project fear”, which he claimed had been “consistently wrong”, adding that his article made clear that the “barriers to trade that the chancellor is fretting about would be against World Trade Organization rules, while he ignores the benefits of future trade deals”.

The clash highlights the looming problems that Johnson is likely to have as prime minister if he wins the Tory leadership contest next week.

The frontrunner has said he wants a deal with the EU that scraps the Northern Ireland backstop by the end of October, but if that fails, many of his Eurosceptic backers are keen to proceed to a no-deal Brexit.

The Telegraph reported on Wednesday that Johnson has appointed the hardline Eurosceptic Daniel Moylan as a senior Brexit policy adviser. Moylan, who worked with Johnson early in his political career, has criticised the promise of “no infrastructure” at the Irish border after Brexit and argued Dublin should accept “a minimally visible trade border”.

Hammond is one of many senior Tories on the other wing of the party who will fight hard against a no-deal outcome, with the chancellor even suggesting he could vote down his own government to stop it.

Thursday’s crunch Commons vote is set to expose those divisions in the House of Commons, which are likely to plague an incoming Johnson administration. On Wednesday, peers passed beefed-up protections against a new prime minister attempting to prorogue parliament to force an exit from the EU on 31 October.

The amendment, which was backed by 13 rebel Tory peers, builds on an amendment to the bill passed in the Commons last week, to mandate fortnightly reports from the government on progress being made towards restoring the power-sharing executive in Northern Ireland.

The new Lords amendment would ensure these have to be debated in the weeks before the Brexit deadline, theoretically making it illegal for the government to prorogue parliament in the autumn if the power-sharing executive has not been restored.

Though it passed the House of Lords by a hefty majority of 103, the previous amendment by Dominic Grieve passed by just one vote – after a government whip forgot to vote.

In her speech, May refused to criticise Johnson’s approach directly, but suggest that refusal to shift from a rigid ideology was the cause of the UK’s current predicament.

Asked whether she would rule out that she was talking about Johnson or the US president Donald Trump, she called the remarks “a general observation I have seen in politics across the world … I wanted to speak about it today as I think it is important to bring debate back into a more reasoned area so that people holding different views can have serious debate and discussion with each other.”

May said when she had brought her deal to parliament, MPs were already entrenched in their pre-referendum positions and railed against the coarsening of language in politics, highlighting that “ill words that go unchallenged are the first step on a continuum to ill deeds, towards a much darker place where hatred and prejudice drive not only what people say, but also what they do.”

“Some are losing the ability to disagree without demeaning the views of others,” she said, highlighting the abuse of female MPs in particular. “This descent of our debate into rancour and tribal bitterness, and in some cases even vile abuse at a criminal level, is corrosive to the democratic values which we should all be seeking to uphold.”

The departing prime minister also said she regretted making mistakes in her own language, particularly having referred to EU migrants as “queue-jumpers”, when confronted over accusations that she had contributed to demonising language.

Following her criticism of Trump for using racist language towards US congresswomen, May took aim at populism across the globe.

“This absolutism is not confined to British politics,” she said. “It festers in politics all across the world. We see it in the rise of political parties on the far left and far right in Europe and beyond.

“And we see it in the increasingly adversarial nature of international relations, which some view as a zero-sum game where one country can only gain if others lose. And where power, unconstrained by rules, is the only currency of value.

“This absolutism at home and abroad is the opposite of politics at its best. It refuses to accept that other points of view are reasonable. It ascribes bad motives to those taking those different views.”

Labour MP David Lammy described May’s speech as “hollow words”, referencing the controversial Home Office advertising campaign of 2013 that encouraged illegal immigrants to “go home or face arrest”.

“Hollow words from a PM who spent her years in office promoting Nigel Farage’s agenda with populist rhetoric, insulting EU citizens as ‘queue-jumpers’, and saying internationalists are ‘citizens of nowhere’,” he said. “We’ll never forget the ‘Go home’ vans you put on our streets.”