Belfast born Conservative MP Conor Burns has been sacked as Trade Minister following a complaint of "serious misconduct" at this week's party conference in Birmingham.
A No 10 spokesperson said: "Following a complaint of serious misconduct, the prime minister has asked Conor Burns MP to leave the government with immediate effect.
"The prime minister took direct action on being informed of this allegation and is clear that all ministers should maintain the high standards of behaviour – as the public rightly expects."
Known as a close ally of Boris Johnson, Mr Burns (50) has also had the party whip suspended, and in a statement told the Sun he would "fully cooperate with the (party's) investigation and looks forward to clearing his name."
A statement from the Conservative whips' office said: "We have suspended the whip pending investigation into allegations of inappropriate behaviour earlier this week. We take all such allegations extremely seriously. The prime minister has been clear that the highest standards in public life must be upheld."
The development follows several high profile scandals of inappropriate behaviour in the party.
This year, Chris Pincher lost the party whip after being accused of drunkenly groping two men in a club while an anonymous Tory MP has been released on bail following an allegation of rape.
The Conservative MP Neil Parish also resigned after being caught watching pornography in the House of Commons.
Born in Belfast in 1972, Conor Burns moved with his family to Hertfordshire at a young age and was first elected as MP for Bournemouth West in 2010.
He previously served as Trade minister between 2018-20, resigning following a House of Commons probe into claims he threatened a company chairman about a financial dispute with his father.
He later returned to cabinet serving as Northern Ireland Minister and again as Trade Minister.