* Lawmakers say MI5 made mistakes made over 2017 bomber
* Flaws over monitoring, travel restrictions
* Issues raised are similar to those after previous attacks
* Businesses should lean on social media firms, lawmakers say (Adds details, quotes)
By Michael Holden
LONDON, Nov 22 (Reuters) - Britain's MI5 security service missed potential opportunities to prevent a 2017 suicide bombing at a pop concert that killed 22 people, senior lawmakers said in a report on Thursday, adding that the government had failed to learn from previous attacks.
Britain suffered four attacks last year that killed 36 people, the most deadly of which occurred at the end of an Ariana Grande show in Manchester, northern England, in May.
In a report, parliament's Intelligence and Security Committee (ISC) said there had been a number of failures by MI5 over the Manchester bomber, 22-year-old Briton Salman Abedi.
"While it is impossible to say whether these would have prevented the devastating attack on May 22, we have concluded that, as a result of the failings, potential opportunities to prevent it were missed," said Dominic Grieve, the committee chairman.
Britain is currently on its second-highest threat level of severe, meaning an attack is considered highly likely and security officials say they are facing record levels of work in countering Islamist militants as well as far-right extremists.
Whilst acknowledging their heavy workload and many other successes, the ISC said MI5 had made mistakes over Abedi who had been known to the security agency since 2014.
He visited a jailed miliant in prison but no follow-up action was taken by police or MI5. Shortly before he carried out the bombing his case had been flagged for review, but "as MI5 put it, 'the plot then moved faster than the process,'" the report said.
He was never referred to a counter-extremism programme and MI5 also did not restrict his travel, allowing him to return to Britain undetected in the days before he carried out the attack.
"MI5 have admitted that, given the information they had on Abedi, they should have done so," Grieve said. He added there was another issue which caused great concern but could not be publicly commented on because it would cause "serious damage" to national security.
Grieve said it was striking how many issues had been previously raised in ISC reports on other attacks, including the July 2005 London public transport bombings which killed 52 people.
"We previously made recommendations in all of these areas yet the government failed to act," he said.
MI5 actively monitors 3,000 "subjects of interest" (SOI) while there were another 20,000 who have been investigated and had their case closed because there was no intelligence to show involvement in attack planning. Abedi had been a "closed SOI"
"Following the attacks, the government, police and MI5 undertook a series of rigorous reviews to ensure we are all doing everything we can to tackle the evolving threat of terrorism," Home Secretary (interior minister) Sajid Javid said.
Britain's top counter-terrorism police officer said the authorities had thwarted 13 other plots since March 2017. (Editing by Stephen Addison and Angus MacSwan)