Police and MI5 could have prevented the Paris and Brussels terrorist atrocities after a secret operation targeting British suspects linked to the plot was launched months before the attacks.
But senior officers decided to halt the undercover probe three months before one of the coordinators of the Paris attacks crossed the Channel to meet extremists in Britain, it can now be disclosed.
The decision meant that investigators missed a key meeting between a group of alleged Islamic State supporters in Britain and Mohamed Abrini last summer – which could have led them to the Brussels-based cell before they carried out a wave of shootings and bombings, killing 130 people in Paris last November.
Dubbed the ‘man in the hat’, Abrini was later caught on camera fleeing Brussels airport after the bombing in March and he is accused of coordinating both the Paris and Brussels attacks.
The crucial missed opportunity has come to light as a result of extensive inquiries by British and European security officials in the wake of the Paris and Brussels attacks.
It can now be revealed that police had arrested and questioned a number of British individuals in early 2015 as part of an investigation into ISIS supporters here in the UK.
Prior to their arrest, the suspects had been the subject of an undercover probe by police and the intelligence services.
But the covert operation ended when they were held by police on suspicion of terrorism offences.
Following extensive investigation, it was decided that there was not enough evidence to bring charges against the alleged British cell and the probe was later dropped.
By the time Abrini visited Britain a number of months later in Summer 2015 police and MI5 were no longer watching the British group.
To make matters worse, following their arrest the suspects had adapted complex security measures to evade the authorities.
They regularly changed phones, deleted data and communicated in internet cafes as they knew they could be under surveillance, according to sources familiar with the inquiry.
Mohamed Abrini travelled to various locations during his seven day trip to the UK in the summer of 2015.
He was later found with images on his phone showing locations he had visited.
During his visit, he stayed in hotels in three cities, but it is thought that the trip was intended to meet individuals here.
It is understood that British detectives have now been able to interview Abrini who has made a full confession, detailing his movements in Britain.
The disclosures are bound to raise questions over whether MI5 and counter-terrorism police did enough to follow up the British connection which could have led them to the Paris plotters.
Sources say the original MI5 operation conducted in early 2015 targeted a group believed to be planning to travel to Syria to join ISIS after at least three other members of the group had already disappeared.
As part of that investigation, they discovered letters which they believed connected the British individuals they had arrested to members of ISIS.
But the information was not enough to bring charges so police were forced to release their suspects.
Security sources argue that substantial efforts were ploughed into the group but there was ‘nothing to suggest that they merited 24-hour surveillance’.
Without such measures it was ‘almost impossible’ to have spotted Abrini’s arrival in Britain and the activity that followed, sources say.
Abdelhamid Abaaoud, who had returned from Syria to plan the attacks, was a childhood friend of Abrini from the Molenbeek area of Brussels.
Had Abrini been followed back to Belgium the cell is likely to have been discovered.
On November 11 last year, Abrini drove former bar-owner Salah Abdeslam, another of the Molenbeek friends, to Paris as they prepared the attacks two days later which killed 130 people.
Abaaoud, the ring-leader, was killed in a shootout in Saint-Denis, Paris, five days later, along with his cousin, the former party girl Hasna Ait Boulahcen, and Chakib Akrouh, who blew himself up.
Abdesalam, who had driven the attackers around and booked their hotels, was tracked down to a flat in Molenbeek on March 18 this year and arrested.
His brother, Brahim, had died in the Paris attacks.
Abrini remained on the run and went on to appear alongside bomb-maker Najim Laachraoui and Ibrahim el-Bakraoui at Brussels Airport four days later as they launched attacks that killed 32 people.
He was finally arrested on April 8 in Anderlecht, Belgium.