Britain's largest police force is putting children at risk because of "serious errors" in its response to child sexual abuse, according to inspectors.
The Metropolitan Police were found to be inconsistent in their handling of child abuse and sex exploitation cases and had a lack of leadership, HM Inspectorate of Constabulary (HMIC) said.
A review of 374 cases found 278 - or three quarters - were handled inadequately or required improvement and 38 were referred back to the force as there was a continued risk to the child.
HM Inspector of Constabulary Matt Parr said: "We met many officers and staff in the Met who are dedicated men and women, working hard to prevent children from coming to harm. But we found serious errors of judgment, inconsistency, unacceptable delays and a lack of leadership which meant that children are not being protected properly."
The report comes weeks after a damning review found "numerous errors" in Scotland Yard's Operation Midland probe into claims of a VIP paedophile ring.
In 132 of the cases audited, HMIC judged 76% to be inadequate or requiring improvement but the Met's own assessment deemed around 80% to be good or adequate.
In one case, a 14-year-old girl disclosed she had been given alcohol and cigarettes before being sexually assaulted by a 30-year-old man she met online.
An officer was not assigned to the investigation until 17 days later, during which time the man continued to message the teenager, and HMIC said this put her at risk while the Met rated their handling as good.
The Met's IT systems for storing information on those at risk of abuse was "fragmentary" and there was a backlog of visits to registered sex offenders, including those who pose a very high risk to children. And HMIC found that of 40 custody cases, 39 resulted in children being kept in cells.
The Met Police were also the first force to be inspected which did not have a dedicated chief officer responsible for child protection, HMIC said.
The HMIC report made recommendations for immediate improvements, including London-wide oversight of child protection to ensure consistency, and the force will face another inspection in 12 months' time.
In response to the report, Mayor of London Sadiq Khan set out a programme of action including the creation of a new independent group of child protection experts and academics, and regular oversight of the issue in his formal meetings with senior Met figures.
The force said that since the HMIC inspection it has revisited all the cases the watchdog examined. It said: "We have identified no further harm to children and no further offenders have been charged or cautioned as a result."
Home Secretary Amber Rudd said she had requested public quarterly reports from HMIC on the Met's progress.