Damian Green, one of Theresa May's closest allies, has been sacked from the cabinet after an inquiry found he had breached the ministerial code.
He was "asked to quit" after he was found to have made "inaccurate and misleading" statements about what he knew about claims pornography was found on his office computer in 2008.
He also apologised for making writer Kate Maltby feel uncomfortable in 2015.
Laura Kuenssberg said the PM "had little choice but to ask him to go".
The BBC's political editor said the departure of a close friend left Mrs May a "lonelier figure".
Mr Green, 61, who as first secretary of state was effectively the PM's deputy, is the third cabinet minister to resign in the space of two months, Michael Fallon and Priti Patel having both quit in November.