Sir Alan Duncan has stopped against a Boris Johnson success in the leadership race.
Justice Secretary David Gauke and chancellor Philip Hammond have said when he wins, they wish to resign.
BBC helper editor Norman Smith stated they couldn’t stomach the possibility.
However he explained the movement of Sir Alan wasn’t predicted to be the first of several.
Voting for the pioneer closes at 17:00 BST together with the winner to be shown on Tuesday.
In his resignation letter to Theresa May, Sir Alan said that it was”dreadful” her administration was dominated by”the dark blur of Brexit” – that he said had ceased the UK getting the”dominant political and intellectual force” on earth.
He commended Mrs May for her”faultless dignity along with an unstinting awareness of responsibility”, adding that she”deserved better” than to possess her period in office” brought to a conclusion” in these conditions.
Sir Alan also spoke his own album in the Foreign Office from the letter, also stated he remained”deeply upset that some profitable discussions I’d initiated concerning the potential launch of Nazanin Ratcliffe were attracted to this abrupt stop”.
As Foreign Secretary, Mr Johnson was nominated because of his handling of the case of Ms Zaghari-Ratcliffe, a woman serving a sentence in Iran.
Theresa May thanked Sir Alan to”the service you’ve shown me, not only during the previous 3 decades, but over many years we’ve understood each other”, and commended his”dedicated and playful service”.
Most recently, his boss was assaulted by Sir Alan to the US, who resigned following remarks were leaked on the resignation of Sir Kim Darroch.
Sir Alan said Mr Johnson – by neglecting to give his assistance to the ambassador – had”essentially chucked our high diplomat under the bus”.
He has also already said Mr Johnson has been”the last man on Earth who’d make any advancement in negotiating with the EU in the moment”.
In 2018 he explained a post – where Mr Johnson stated Theresa May’d “wrapped a suicide vest” across the British ministry – as”among the funniest moments in contemporary British politics”.