Families affected by the Manchester Arena bombing have said MI5 failed them and must be more open to scrutiny.

In a letter sent to the prime minister seen by the BBC, the families demanded the Security Service be fully included in a new law designed to stop cover-ups in public life.

The inquiry into the bombing found it could have been stopped if MI5 had acted on crucial intelligence in the months before the attack.

“How many times must MI5 show that it cannot be trusted before something is done?” the letter reads.

Keir Starmer’s government 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 organisations.

Salman Abedi killed 22 people by detonating a homemade rucksack bomb in a suicide attack at an Ariana Grande concert at the venue in May 2017.

His brother Hashem Abedi, who was convicted of assisting with the terror plot, was jailed for life with a record-breaking 55-year minimum term in August 2020.

In December, a judge ruled that almost £20m is to be paid out to children injured in the bombing.

Amounts ranging from £11.4m to £2,770 were agreed at a hearing at the Manchester civil courts of justice for 16 victims, all aged under 16 at the time of the attack, with the total amount to be paid coming to £19,928,150.

Some suffered “catastrophic” and life-changing injuries, while others suffered psychological damage from witnessing the carnage.

Sir John Saunders, the chair of the Manchester Arena inquiry, said in March 2023 the failure of MI5 to act swiftly on crucial intelligence was a “significant missed opportunity” to take action that might have prevented the atrocity.

There was a “realistic possibility” that investigators could have thwarted the bomb plot had they acted more decisively on two pieces of intelligence in the run-up, he concluded. The report did not describe the nature of the intelligence.

The retired high court judge said MI5 would have taken “extremely seriously” Abedi’s return from Libya, where he had been fighting alongside Islamists, had they taken firmer action on this intelligence.

Abedi returned from Libya four days before the attack and used this time to construct the bomb while trying to evade surveillance.

Saunders said tougher action could have led to the attacker being followed to the Nissan Micra where he had stored his homemade explosives.

Abedi went to check on the car just over an hour after returning to the UK. He then checked in to the rented city centre flat where he built the device, before scoping out the arena.