MP Conor Burns has been sacked from the Government following a complaint of “serious misconduct”.
The former trade minister has also had the Tory whip suspended pending an investigation into “allegations of inappropriate behaviour” earlier this week.
The MP for Bournemouth West, who has held the seat since 2010, has said he will co-operate fully with the probe and “looks forward to clearing his name”, according to The Sun.
Mr Burns was appointed minister of state in the Department for International Trade in last month’s reshuffle, after Liz Truss took the reins in No 10.
He was previously a minister in the Northern Ireland Office, and had been a staunch ally of former prime minister Boris Johnson.
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.”
A spokesman for the 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 PA news agency has approached Mr Burns for comment.
Government minister Victoria Prentis said the allegations were “obviously concerning”, but she had been assured the matter was being taken “very seriously”.
“I think all I can say is that the Prime Minister has taken decisive action, and we’ll make sure that this is properly investigated,” she told Times Radio.
“In terms of politics as a whole it’s always worrying because we all expect and hope to maintain the highest standards in public life.”
The development is the latest in a series of scandals to rock the Conservative Party, including groping allegations against former deputy chief whip Chris Pincher.
In May, former Tory MP Neil Parish quit his seat in Tiverton and Honiton after admitting to watching pornography in the Commons.
Just short of two weeks later, an unnamed Conservative MP was arrested on suspicion of rape and sexual assault offences spanning seven years.