"Damian Green, one of Theresa May's closest allies, has resigned from the cabinet after an investigation found he had breached the ministerial code.
He quit after he was found to have made "inaccurate and misleading" statements about what he knew about claims pornography had been found on a computer in his Commons office in 2008.
In his resignation letter, Mr Green apologised for his actions.
BBC's Laura Kuennsberg said the PM "had little choice but to ask him to go".
In her written response, Mrs May expressed "deep regret" at his departure.
Mr Green, who as first secretary of state was the PM's deputy, had been under investigation regarding allegations of inappropriate conduct.
He denied suggestions that he made unwanted advances to a female journalist, Kate Maltby, in 2015 and viewed pornography on a computer in his Commons office in 2008."