One of the London Bridge terror attackers was allowed to enter the UK despite Britain's intelligence agencies being told he wanted "to be a terrorist".
Youseff Zaghba was placed on an international 'watchlist' of suspected foreign fighters after the Italian police caught him trying to travel to Syria last year.
The Italians claim both MI6 and MI5 were informed of the fears surrounding Zaghba, who told police he was "going to be a terrorist" when he was stopped at Bologna airport.
But the 22-year-old Italian national was still able to enter the UK and went on to join Khuram Butt and Rachid Redouane in Saturday’s van and knife rampage in central London.
As the row threatened to overshadow the final days of the general election, Theresa May was facing questions over the actions of the intelligence agencies and the Home Office at a time when she was Home Secretary.
During a campaign rally in Slough she announced plans to sidestep human rights laws to toughen controls on terror suspects by tightening limits on their internet use and increasing curfews
The Foreign Secretary, Boris Johnson, said the public was entitled to wonder "How on earth" the security services failed to allow Butt "through the net" after he appeared on a documentary entitled "The Jihadis Next Door" yet was still deemed low risk.
It was also revealed that Butt later received a caution for assault after attacking a man in a row over Islam.
In further disclosures, it emerged that Redouane was refused asylum in Britain in 2009 but was seemingly able to stay on in the country.
It came as:
Scotland Yard named Zaghba as the third and final attacker but reports from Italy immediately revealed that security services knew of his attempts to travel to Syria in an apparent effort to join the Islamic State of Iraq and the Levant (Isil).
Lord Carlile, the former Independent Reviewer of Terrorism Legislation, said:
We should have turned him away. People have been excluded on the grounds of not being conducive to the public good for far less.
Zaghba was a danger. There were powers there to exclude him and they have not been used.
It was also disclosed that Butt, the attack ringleader, was arrested by police earlier this year after he attacked an Islamic scholar - but he was let off with a caution.
He was reported to counter-terrorism officers following the incident but they concluded he did not pose a terrorist threat.
Libyan security sources, meanwhile, said they believed the other attacker, Redouane, had fought in the revolution against Col Muammar Gaddafi and joined a militia brigade that went on to send foreign fighters to Syria.
The Prime Minister on Tuesday said she expected the security services to launch a review into their handling of the London Bridge attack.
She said: "MI5 and the police have already said they would be reviewing how they dealt with Manchester and I would expect them to do exactly the same in relation to London Bridge."
Zaghba, the son of an Italian mother and Moroccan father, was arrested at Bologna airport on March 15 last year carrying a one-way ticket to Istanbul and a small rucksack.
The Italians suspected that he was on his way to Syria to volunteer as a foreign fighter with Isil or another terrorist group.
The police confiscated his passport and mobile phone, on which they found videos and images related to Islamic jihad, including the black Isil flag and a video clip of prisoners in orange jumpsuits being decapitated in a desert setting.
When he was stopped and questioned, he reportedly told police officers: “I’m off to become a terrorist”.
Zaghba’s mother said her son had become radicalised while working in a Pakistani restaurant in London.
A court ruled there was not enough evidence to charge him with terrorist offences, but Italy’s Internal Intelligence Service reportedly passed on details of the airport incident to the MI6 liaison officer in Italy, who also relayed the information to MI5.
Giuseppe Amato, a prosecutor who dealt with the case, said: “His computer was confiscated, but according to the court, there was no evidence to suggest any crime had been committed and it was given back to him.
He was flagged up to London as a possible suspect. In a year and a half he was in Italy for just 10 days and he was always followed by special operations police.
His name was also added by the police in Bologna to a European Union security database called the Schengen Information System.
Britain signed up to the system two years ago, with the Home Office saying it would send law enforcement alerts on those including “suspected terrorists”.
EU security sources said the system should have triggered an alert to border officials when Zaghba crossed into the UK.
Scotland Yard said Zaghba had not been a police or MI5 “subject of interest” before the attack.
Khalid Mahmood, Labour's parliamentary candidate for Birmingham Perry Barr, who has repeatedly complained over the ease with which jihadists can come and go from the UK, said: "Our borders are like a sieve. Theresa May when Home Secretary dramatically cut the numbers of border staff.
It is easy for jihadists to come in with biometric passports and use the machines at the airports. As I understand it the biometric machines are not linked to the various watchlists being used. The system is not coordinated.