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 was found to be inconsistent in its 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 its 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 of 40 custody cases, 39 resulted in children being kept in cells.
The Met was also the first force to be inspected which did not have a dedicated chief officer responsible for child protection, HMIC said.
Mr Parr said: "This absence of oversight of this crucial area is unacceptable and exacerbates the inconsistency we found in dealing with child protection.
"Far too many of the cases we looked at fell well short of expected standards and meant that victims weren't protected, evidence was lost and offenders continued to pose a risk to children."
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.
HM Chief Inspector of Constabulary Sir Thomas Winsor said: "Of course, child protection is not the sole preserve of the police, but we found inexcusably poor practice at every stage of a child's interaction with the police and across the parts of the force we inspected.
"The Met is a large and complex force but, nevertheless, the importance of the police getting this right and protecting children cannot be overstated."
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 Mayor said: "This deeply troubling report has found that, too often, children in our city have been let down when they are most in need. This is simply unacceptable and things must change."
Scotland Yard said it aims to provide the best possible protection to children, adding that it was "sorry that this has not always been the case, especially, to the children involved in the examples highlighted in this report".
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."
Assistant Commissioner Martin Hewitt said: "Child protection often involves complex social problems which cannot be solved by the police alone, however we have an important role to protect them from further harm as often we arrive in a child's life when they are already in crisis.
"We have re-thought how we go about our work to protect London's most vulnerable.
"As this report shows, there is still a lot of work to be done, we have made significant changes already and we are committed to continuing to rise to this challenge."
Home Secretary Amber Rudd said she had requested public quarterly reports from HMIC on the Met's progress.
She said: "The findings of this report are profoundly concerning. They raise serious issues that must be urgently addressed."