An inquest into the killing of Azezur Khan, 21, at a cemetery in south-east London heard that police were asked to attend by the mother of Joel Morgan, the 17-year-old being buried.
However no officers were assigned.
An inquest jury at Southwark Crown Court said local police failed to liaise and share information.
Returning a narrative verdict, a jury spokesman said the force had failed to recognise "that you don't need a named perpetrator and named victim for there to be a threat to life".
Mr Morgan, who was a member of the Lambeth GAS gang, was killed in a car crash.
His mother had asked police to attend the funeral because it was in a patch controlled by two rival gangs, the Peckham Young Guns and the Peckham Boys.
His friend Mr Khan was in Forest Hill Road outside Camberwell Old Cemetery when he was caught in an exchange of gunfire.
The killers have not been found.
The jury spokesman said: "If all relevant information regarding the funeral and gang activities were linked, the Metropolitan Police Service would have put in place a different policing plan, raising the level to either a higher risk or critical.
"If there was a visible police presence at or in the vicinity of the burial service, it may have deterred gang members."
He said communication errors were "prevalent" and officers failed to liaise with each other to carry out their policing plan.
Coroner Andrew Harris said he could find no systemic failings in the Met's intelligence gathering systems, but he did question its policy on threat to life.
He said: "I'm not clear that we can be confident that the Met Police has addressed this policy, and that is a considerable concern.
"Given the level of violence in this borough, I can't be satisfied that this has been dealt with."
A Met spokeswoman said it acknowledged the jury's findings.
"Similar issues were highlighted by an independent investigation conducted by the Independent Police Complaints Commission, and their recommendations were acknowledged and implemented in 2012.
"Since Mr Khan's murder there has been considerable change in the way the Met deals with intelligence.
"We will now take time to consider in detail the findings of the inquest."

Helen Rossenberg worked for Bourne End Academy in Buckinghamshire from September to December 2015.
A conduct panel heard the 59-year-old pushed one student, grabbed another by the hair and trod on a third's foot.
The panel also found an allegation she referred to a student with special needs as "thick" proven.
Ms Rossenberg did not attend the National College for Teaching and Leadership hearing.
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It is understood Ms Rossenberg accepted she made a comment that had some absent students been present in class "all they would do is sit there scratching their balls".
In banning Ms Rossenberg from teaching, the panel found "there had been a rapid escalation in the seriousness of the incidents that pupils were exposed to by Ms Rossenberg's behaviour, both oral and physical".
An inquest jury has criticised the Metropolitan Police for failing to send officers to a funeral where a young mourner was shot dead in 2011.
[NEXT_CONCEPT]
A teacher who claimed all some pupils would do in a class is "sit there scratching their balls" has been banned from the profession.