Alan Duncan has resigned as a Foreign Office minister ahead of Boris Johnson’s expected victory in the Tory leadership contest.
The longtime critic of the incoming Tory leader once described the former foreign secretary as a “circus act”.
Duncan is likely to be the first of many ministers to quit the rather than serve under Johnson’s leadership or be fired.
Chancellor Philip Hammond and Justice Secretary David Gauke have both confirmed they will step down before Johnson becomes prime minister on Wednesday afternoon.
In his resignation letter, Duncan said: “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.”
Sir Alan, a university friend of May, praised the “faultless dignity and an unstinting sense of duty” she had shown.
“I am only sorry that your three years as prime minister have been brought to an end,” he said.
Johnson is expected to easily defeat Foreign Secretary Jeremy Hunt in the leadership contest, with the result set to be announced on Tuesday morning.
Theresa May will take her final PMQs in the Commons on Wednesday before heading to Buckingham Palace resign as prime minister.
The Queen will then meet the new leader of the Tory Party at the Palace and invite him to form a new government.
Traditionally, the new prime minister makes a statement in Downing Street before stepping into No.10 for the first time.
Johnson will likely spend his first full day in office finalising his government - with appointments to the cabinet and ministerial team expected.