Britain’s biggest police force has abandoned its policy of automatically believing victims after a series of flawed inquiries into alleged sex crimes, The Times can reveal.

Cressida Dick, the Metropolitan Police commissioner, said she had told officers they must have an open mind when an allegation is made and that their role was to investigate, not blindly believe.

“You start with a completely open mind, absolutely,” she said. “It is very important to victims to feel that they are going to be believed. Our default position is we are, of course, likely to believe you but we are investigators and we have to investigate.”

The issue has become one of the most fraught for the police service since a national policy instructed officers to believe