The bombing at an Ariana Grande concert in Manchester that killed 22 people might have been averted had British security services handled intelligence about the attacker differently, according to a new government report.

MI5 had information about the suicide bomber, Salman Abedi, “whose true significance was not appreciated at the time,” according to an official report, the Guardian reported.

The domestic security agency had planned to discuss the threat posed by Abedi during a meeting on May 31 — but he detonated his explosives at the Manchester Arena nine days earlier.

A report by lawyer David Anderson said that in retrospect, different decisions might have been made, but it was unknowable if Abedi would have been stopped.

Anderson, a former independent reviewer of terrorism legislation, oversaw reviews of four UK terrorist attacks between March and June to provide assurance to the government that the internal reviews by police and MI5 were thorough enough.

The four attacks killed a total of 36 people and wounded 200 others.

He said decisions made by counter-terrorism officials were mostly justifiable, but his report highlighted questions for MI5 and police as they battle a mounting terrorist threat.

On two occasions, MI5 had received intelligence on Abedi “whose significance was not fully appreciated at the time,” Anderson said, according to the Telegraph.

“In retrospect, the intelligence can be seen to have been highly relevant to the planned attack,” said Anderson, whose 61-page report concluded it was “conceivable that the Manchester attack in particular might have been averted had the cards fallen differently.”

The report found that Abedi, 22, who was born in Manchester to Libyan parents, had first become an MI5 “subject of interest” in 2014, but his case was closed after he was mistaken for someone else, the Telegraph reported.

It was reopened in 2015 on mistaken intelligence that he had contacted an ISIS figure in Libya.

“Salman Abedi continued to be referenced from time to time in intelligence gathered for other purposes,” according to the report.

In the run-up to the May 22 attack, intelligence was received that was “assessed at the time to relate not to terrorism, but to possible non-nefarious activity or to criminality.”

An automated sweep of suspects’ data designed to identify closed cases that might need re-examining “identified Salman Abedi as one of a small number of individuals, out of a total of more than 20,000 closed subjects of interest, who merited further examination.”

“A meeting (arranged before the attack) was due to take place on May 31: Salman Abedi’s case would have been considered, together with the others identified,” according to the report.

Authorities failed to stop Abedi when he returned to the UK from Libya, according to the findings.

“With the benefit of hindsight, intelligence was misinterpreted in early 2017,” Anderson said.

The report was published shortly after the cabinet was briefed by MI5 chief Andrew Parker, who said nine terrorist attacks had been prevented in the past year.

A review showed three of six attackers “were on MI5’s radar.”

As well as Abedi, Khurram Butt, one of the London Bridge attackers, was under active MI5 investigation, while Khalid Massood, the Westminister Bridge attacker, also was a former subject of interest.

Butt was the “principal subject” of an MI5 probe called Operation Hawthorn that “opened in mid-2015 following information suggesting that he aspired to conduct an attack on the UK,” according to the Telegraph.

He was placed under extensive surveillance, but authorities eventually deemed his focus to have moved away from attacking the UK and toward traveling abroad to fight for ISIS.

It also was disclosed that Masood, 52, had watched suicide attack videos on YouTube in the days before he mowed down pedestrians on March 22, killing four, and then stabbed a cop to death.

“The excellent recent record of MI5 and police in defending the UK from terrorist attack came to a brutal end this year at Westminster, Manchester Arena, London Bridge and Finsbury Park,” Anderson said.

“After four such incidents over a short period, unsparing reflection was required.”