MI5 can’t be trusted to prevent another Manchester Arena bombing, victims’ families have told PM Sir Keir Starmer.

In a letter sent to the Prime Minister, seen by the BBC, they demanded the security service be fully included in a new law designed to stop cover-ups in public life.

The inquiry into the atrocity heard it assessed that key intelligence months before Salman Abedi’s suicide bomb attack did not relate to terrorism.

Chairman Sir John Saunders said that this was not an “accurate picture” after hearing from MI5 witnesses.

Abedi was never subject to controls on his movements to Libya despite a stream of intelligence showing his contacts with extremists before he murdered 22 people at an Ariane Grande concert in May 2017, the inquiry heard.

He, his father Ramadan and brothers Ismail and Hashem were allowed to freely operate in the war zone for years despite both Ramadan and Ismail being subject to port stops in Britain in 2011 and 2015, respectively, by security services, who downloaded extremist material from the Abedis’ mobile devices.

The security services reportedly encouraged British-Libyan radicals to fight against Muammar Gadaffi’s government during Nato’s attack on Libya in 2010.

Director of the Hillsborough Law Now campaign Pete Weatherby KC today told BBC Radio 4’s Today programme: “If this law is passed and they’re required to tell the truth even when things go wrong, then failures can be rectified and people can be safer in the future.”

Campaigners understand that some evidence cannot be aired in public because of national security, he told the broadcaster.

Sir Keir Starmer has introduced a so-called Hillsborough Law in Parliament — the Public Office (Accountability) Bill — which will force public officials, and contractors, to tell the truth in the aftermath of disasters, and to investigate bodies.

Deborah Coles, director at the charity Inquest that supports families who have lost loved ones in state-related deaths, said: “The duty of candour must apply in full to security and intelligence services and every public service so that no institution can hide behind secrecy.”

Downing Street said: “The Hillsborough Law will apply to all public authorities, including the intelligence agencies… while also protecting national security.”