Britain's MI5 Security Service has started an internal review on whether there are lessons to be learned from its handling of intelligence on Salman Abedi before he killed 22 people by blowing himself up at a concert in Manchester.

"This is a review that would seek to answer whether there are lessons to be learned from how the Security Service handled the intelligence on Abedi," the source, who spoke on condition of anonymity, told Reuters.

Before he carried out the attack, Abedi, a British-born Muslim whose parents were from Libya, was one of 20,000 people who was known to MI5 but not one of the 3,000 people who are under active investigation, according to the source.

MI5, established in 1909 to counter German espionage ahead of World War One, is tasked with protecting British national security. Most of MI5's work is focused on counter-terrorism and it employs around 4,000 people.