Foreign Office minister Alan Duncan, who quit on Monday, said Britain was being forced to work under a "dark cloud" caused by Brexit, setting out the reasons for his resignation in a letter to Prime Minister Theresa May.
"It is tragic that just when we could have been the dominant intellectual and political force throughout Europe, and beyond, we have to spend every day working beneath the dark cloud of Brexit," he wrote in the letter, published on Twitter.
His move follows last week's resignation of Margot James, a culture minister when she described Johnson's do-or-die promise to leave the European Union (EU) by October 31 with, or without, a deal as "quite incredible" for going against business organisations.
If the polls and bookmakers are right, Johnson will become Britain's new prime minister on Wednesday and will immediately face the riddle that is Britain's Brexit negotiation.
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.
Meanwhile, Johnson's leadership rival foreign minister Jeremy Hunt attended the British government's emergency response committee Cabinet Office Briefing Rooms (COBR) on Monday to discuss Iran's seizure of a British-flagged tanker.
'Britain was being forced to work under a "dark cloud" caused by Brexit', Alan Duncan.