Sir Alan Duncan has quit as a minister at the Foreign Office, ahead of Boris Johnson's expected leadership.
The MP for Rutland and Melton handed in his resignation letter to Theresa May this morning.
Sir Alan, who has served as Tory MP for Rutland and Melton since 1992, was appointed Minister of State at the Foreign and Commonwealth Office on 17 July 2016.
Sir Alan's exit was confirmed by Downing Street sources.
It comes after Chancellor Philip Hammond and Justice Secretary David Gauke have already announced their intention to resign this week.
Mr Johnson is widely expected to defeat Foreign Secretary Jeremy Hunt to become the new Conservative leader on Tuesday.
Sir Alan has been one of Mr Johnson's fiercest critics on the Tory benches.
His resignation comes after Mr Johnson restated his firm intention to get the UK out of the European Union by the end of October.