Victims of police incompetence and malpractice, including Doreen Lawrence and the brother of murdered private eye Daniel Morgan, have called for the replacement of the Metropolitan police commissioner, Cressida Dick.

In a scathing open letter to Boris Johnson, first reported in the Daily Mail, the panel of influential individuals said that after “decades of equivocation and inertia”, the Met must be held accountable for its failings and Dick should not be handed the two-year extension to her term in office, which she was offered on Wednesday.

Led by Lady Lawrence, the mother of Stephen Lawrence, and Lady Brittan, the widow of the former Conservative home secretary Leon Brittan, the signatories also demand an overhaul of the Met’s senior team, “urgent and long overdue” reform of the police complaints system and a shake-up of the Independent Office for Police Conduct, which was branded “unfit for purpose”.

The letter reads as a vote of no confidence in the beleaguered Met commissioner: “Dame Cressida Dick, who has presided over a culture of incompetence and cover-up, must not have her contract extended and must be properly investigated for her conduct, along with her predecessors and those in her inner circle, who she appointed and who have questions to answer.

“She should be replaced by an appointee from outside London, via a truly independent and transparent process.”

Since becoming Met commissioner in 2017, Dick’s tenure has been controversial and she has often faced accusations of escaping being held properly to account, including for her role in the disastrous Operation Midland VIP paedophile scandal, which wrongly targeted numerous high-profile individuals, such as Leon Brittan, and after the inquiry into Morgan’s murder personally criticised her.

Support for her in Downing Street is said to be lukewarm, and those with knowledge of the process have said she was granted an extension because those most likely to replace her were not yet seen as being suitable for the job.

Calling for an urgent meeting with the prime minister and the home secretary, Priti Patel, the group writes: “We share a collective concern that the leadership of the Metropolitan police service will continue to act as though they are above the law and that the general public do not have a viable means of recourse.”

As well as Lawrence, Brittan and Alastair Morgan, the group also includes the son of D-Day army officer Lord Bramall, BBC broadcaster Paul Gambaccini and the former Tory MP Harvey Proctor.