Children in London are being put at risk because of "serious failings" in the way the Met Police deals with child sexual abuse, inspectors have found.
HM Inspectorate of Constabulary found 75% of cases were dealt with "inadequately" or needing improvement.
It said there was an "indefensible" lack of leadership in child protection, with no single officer in charge.
The Met apologised for any failures but the home secretary described the report as "profoundly concerning".
The force's assistant commissioner Martin Hewitt said the safety of youngsters was a priority but added that child protection "often involves complex social problems which cannot be solved by police alone".
HM Inspectorate of Constabulary (HMIC) inspectors discovered 277 of 384 cases investigated had been poorly handled.
Of those, 38 were referred back to the Met because inspectors believed the children involved may still be at risk.
Examples of 'failings'
- A teenage girl disclosed she was sexually assaulted by a 30-year-old man she met online but it took 17 days for an officer to be assigned to investigate the offence. In that time the man continued to message the girl.
- Posts on a 13-year-old girl's Facebook seen by her family suggested she was being sexually exploited by boys her own age and older men. The Met did not investigate.
- A 17-year-old girl with bipolar disorder, who took medication for seizures and had previously attempted suicide, told her carer she was going out for the night and would not be back. When the carer reported her missing, police marked her as "absent" - meaning no active investigation was launched to find her.
- A suspect who was believed to be making images and movies of child sexual abuse was identified by Met officers in May 2013, but was not arrested until February 2015 despite police receiving further intelligence that he was still actively sharing images.
HM Inspector of Constabulary Matt Parr said "far too many of the cases we looked at fell well short of expected standards".
The force was found to be the only one investigated not to have a chief officer in charge of child protection, in spite of it being the largest police service in the country.
Information about victims and offenders was kept on "isolated" IT systems and not shared between boroughs, while some staff - including borough child sexual exploitation officers - had been given no training in child sexual exploitation.
Mr Parr told BBC Radio 4's Today programme the previous mayor had set the priorities for the Met to tackle crimes such as burglary, criminal damage, theft and violence, which meant it had taken its "eye of the ball" when it came to child protection.
As a result, child protection investigations were "inconsistent" and caused issues such as a backlog of visits to registered sex offenders who were thought to be a very high risk to children.
In a statement, the Met said it already had "more than 1,300 officers" who were "dedicated to protecting vulnerable young people".
It said it had re-examined the cases highlighted by HMIC and had identified "no further harm to children and no further offenders".
With regards to accusations it had focused too much on other areas of policing, the force said "children are disproportionately victims" in those crimes.
However, it said it had "worked closely" with HMIC since the inspection and would use the report as "a launch pad" to change its approach to child protection.
Assistant Commissioner Hewitt told the Today programme: "We had already identified that we needed to change in the way that we managed child protection and we were - as the report acknowledges - we are in the process of a range of activities to change our response to child protection.
"But what this report gives us is some very clear issues that we need to look at and we need to address as we continue in that process of change."
HMIC has made several recommendations, including a London-wide oversight of child protection.
Home Secretary Amber Rudd said: "The findings of this report are profoundly concerning. They raise serious issues that must be urgently addressed.
"I have spoken to the mayor of London and sought his assurance that immediate action will be taken to overhaul the force's approach to child protection."
She added: "Every child has the right to feel safe and protected, and to trust that they can turn to the police for help.
"It is critical that the Met learns from its mistakes and gets this right going forward."
Current London Mayor Sadiq Khan 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."
He sais he has set out a plan to improve the service, including "a new independent group of child protection experts".
Inspectors will carry out another inspection of the force to check on improvements within 12 months.