Boris Johnson knew of "reports, speculation," about the alleged sexual misconduct by Chris Pincher, former deputy chief whip before his February appointment, No 10.
After a complaint that he had groped two men, Mr Pincher has resigned as Tory MP and is now not in Parliament.
He has been subject to a variety of historical claims since then, which he refutes.
Downing Street stated that Mr Pincher was not denied the job due to the unsubstantiated allegations.
Johnson is under increasing pressure to disclose what he knew about Pincher's allegations.
Following claims that Pincher had raped two men last Wednesday, Mr Pincher was resigned as deputy chief whip. He told the prime minister that he had drunk too much.
He was removed as a Conservative MP on Friday. This means that he is currently sitting in Parliament as an Independent.
According to the official spokesperson for the prime minister, Johnson knew about "allegations that were either settled or not advanced to a formal complain", and that it was not appropriate to terminate an appointment because of unsubstantiated claims.
A spokesperson for Mr Pincher stated that he "had many skills and experience for this role", but was asked if Mr Johnson used the phrase "Pincher-by-name, pincher-by nature"? He declined to comment on private conversations.
Nadhim Zahawi, Education Secretary, said that the prime minister did the right thing in handling allegations against Tamworth MP.
Mr Zahawi stated that "there were ofcourse allegations in the past", but Mr Pincher denied them and they were investigated and determined "not to be accurate".
Anneliese Dodds, Labour chairwoman, wrote to the PM to inquire about the details of what No. 10 knew about the claims.
She said that only Boris Johnson could have seen this man's record and concluded, "He deserves a promotion."
Six new allegations of inappropriate behavior against Mr Pincher date back many years. He has denied these claims.
Last week's resignation was his second from the whips' offices. He had previously resigned after Alex Story, a Conservative candidate, accused him of making an inappropriate advancement.
Later, Mr Pincher was cleared of any violation of the Conservative Party code.
Therese Coffey, Work and Pensions Secretary, stated that the prime minister was unaware of any specific allegations against Mr Pincher when he was appointed to his whips' office in Feb.
She stated that she had not spoken directly to the prime minister and that she had received assurances from the No 10 Press Office.
Wednesday, 29 June: Chris Pincher was "extremely drunk", as witnesses told the BBC.
Thursday: After allegations that he had groped two club members, Mr Pincher resigns from his position as deputy chief whip.
Friday: After being reported to Parliament's behavior watchdog, he was suspended as a Conservative MP
Saturday: Mr Pincher said he was seeking professional medical help and that he is truly sorry for the inconvenience caused by him
Sunday: The MP is facing new allegations, which he denies.