LONDON (REUTERS) - A minister and longstanding critic of Boris Johnson quit on Monday (July 22), the latest resignation before the presumed new prime minister takes office with a "do or die" pledge to take Britain out of the European Union with or without a deal.

The resignation of Mr Alan Duncan, a junior foreign office minister, underlines the strength of feeling in the governing Conservative Party and parliament against a no-deal Brexit which many businesses say would be catastrophic for the economy.

He follows Ms Margot James, who stepped down as culture minister last week, describing as "quite incredible" Mr Johnson's promise to leave the EU by Oct 31 regardless of whether a deal was in place to smooth the process.

Business organisations that are traditional allies of the Conservative Party have repeatedly warned against such a scenario.

On Sunday (July 21), finance minister Philip Hammond also said he would resign rather than be sacked by Mr Johnson, promising to fight with others in parliament to stop a rupture in relations with the EU, the country's biggest trading partner.

In his resignation letter, Mr Duncan said: "The UK does so much good in the world. It is tragic that just when we could have been the dominant intellectual and political force throughout Europe, and beyond, we have had to spend every day working beneath the dark cloud of Brexit."

He pointedly noted that he had worked with "two very different foreign secretaries" - Mr Johnson and his rival to become prime minister, Mr Jeremy Hunt.

His decision to step down comes as little surprise. Mr Duncan has shown no reticence in criticising Mr Johnson, his former boss at the foreign office, once describing him as a "circus act".

Earlier this month, he attacked Mr Johnson for not defending Britain's former ambassador to the United States after a leak of his criticism of US President Donald Trump's administration.

Mr Duncan said Mr Johnson had "basically thrown our top diplomat under the bus".

Several other ministers are expected to leave their posts if, as expected, Mr Johnson becomes Britain's new prime minister on Wednesday (July 24). The man who led the "Leave" campaign in the 2016 EU referendum will then immediately face the riddle that is Britain's Brexit negotiation.

Mr Johnson, a former London mayor, has said he will ramp up preparations for a no-deal exit to try to force the EU's negotiators to make changes to the agreement that Prime Minister Theresa May sealed and British lawmakers voted down three times.

But opposition in parliament to leaving without a deal is growing and the EU is refusing to budge over the withdrawal agreement.

On Sunday, Irish Foreign Minister Simon Coveney said "we'll all be in trouble" if the new prime minister wanted to tear up the agreement to try to get rid of the so-called Northern Irish backstop, an insurance policy to prevent a return of a hard border between the British province and EU member Ireland.

"We hope that the backstop that many in the UK don't seem to like can be avoided," Mr Coveney told the BBC. "(But) we are simply not going to move away from that withdrawal agreement."