The Metropolitan Police has been told to make urgent improvements after a watchdog raised "serious concerns" about its performance.
A report published by His Majesty's Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS) said Scotland Yard was "failing" in several areas.
Britain's biggest police force was graded inadequate in the way it responds to the public - amid criticism that emergency calls are not answered quickly enough.
The Met currently answers around 64% of 999 calls within 10 seconds, compared to the national target of 90%.
Some 36.6% of calls to the non-emergency number, 101, are unanswered - against a target of less than 10%.
Scotland Yard was also found to require improvement in investigating crime; protecting vulnerable people; managing offenders; developing a positive workplace and using resources effectively.
HMICFRS also called on Staffordshire Police to urgently improve after pointing to serious concerns over how it investigates crime, responds to the public and monitors suspects and offenders.
Both Staffordshire and the Met are among six police forces put into special measures three months ago by the watchdog, which condemned policing in England and Wales for poor performance.
Concerns growing for a "considerable time"
Inspector of Constabulary, Matt Parr, said concerns about the way the Met operates had been "growing for a considerable time".
The report said officers and staff are not adequately supported, with investigations not always reviewed or overseen properly.
"There is an unfair allocation of work, which puts undue pressure on some staff," Mr Parr said.
A high proportion of inexperienced staff and a lack of experienced tutors for detectives meant supervisors were often teaching people how to investigate crime, rather than overseeing their work.
Mr Khan insisted new leadership was necessary to "rebuild the trust of Londoners" following a series of high-profile failures.
A survey on public attitudes conducted by the London Mayor's Office for Policing and Crime found trust levels dropped from 83% in March 2020 to 73% the following year.
Sir Mark was earlier warned by former home secretary, Priti Patel, that he must address "appalling mistakes of the past".
The force faced fierce criticism for the murder of Sarah Everard by Wayne Couzens, a serving police officer.
It also came under fire for failures while investigating the deaths of the victims of serial killer Stephen Port.
The Independent Office for Police Conduct (IOPC) also raised concerns after investigating more than a dozen cases where a child was strip searched by Met officers.
The force was also accused of being "institutionally corrupt" after "multiple, very significant failings" were found during an investigation into the 1987 axe murder of private detective Daniel Morgan, which remains unsolved.
There has also been criticism over the force's use of stop and search and how it deals with violent crime in the capital - with an investigation launched into the death of Chris Kaba, an unarmed black man fatally shot by police.
Two separate reviews of Scotland Yard are currently taking place including one to scrutinise the force's culture and an inquiry into the police career and conduct of Wayne Couzens.
HMICFRS is also reviewing vetting procedures used by all police forces in England and Wales in the wake of Ms Everard's murder.
"Many successes and examples of innovation"
The Met "operates under scrutiny other forces do not face", the report pointed out.
It was judged to be adequate in two areas of police work but only found to be good in one.
However, the watchdog praised Scotland Yard for the way it handled the Queen's funeral on Monday, which Sir Mark had described as a "massive challenge" for the force.
It was also described as "good at preventing crime and anti-social behaviour" and hailed for developing "innovative techniques" to improve collecting evidence and identifying offenders.
These include a new forensic technique to detect blood on dark clothing and a rapid testing kit for drink spiking.
The force will remain under enhanced monitoring as part of being in special measures.