Another member of Toronto Mayor Rob Ford's staff has left as a controversy swirls about a video that purportedly shows the mayor smoking crack cocaine.

Brian Johnston, the mayor's policy adviser on council relations, became the fourth staff member in less than two weeks to leave Thursday. Johnston says he's leaving on his own accord and the timing is right.

The mayor's press secretary and deputy quit last week. Ford also fired his chief of staff.

The crack-smoking video has not been released publicly, and its authenticity has not been verified. Reports on gossip website Gawker and in the Toronto Star said it was taken by men who said they had sold drugs to Ford.

Ford still hasn't said whether he has ever used crack.