THE secret service is under increasing pressure to explain how the London Bridge terrorists slipped through the net after it emerged two of the three attackers had been known to the authorities.

This means that perpetrators in all three of the terrorist outrages to hit Britain this year had at some point appeared on the radar of the security agencies.

During a visit to North Wales, Prime Minister Theresa May 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."

Yesterday it emerged that Youssef Zaghba, 22, named as the third extremist responsible for Saturday night’s atrocity, had been stopped at at Italian airport trying to fly to Turkey in March last year amid concerns he was intending to travel on to Syria.

He is said to have told authorities in Bologna "I'm going to be a terrorist" while officers reportedly found so-called Islamic State-related material on his mobile phone when he was intercepted.

The Italian national of Moroccan descent was prevented from continuing his journey to Istanbul, placed on a watch list and, it was claimed, flagged up to Moroccan as well as British security services.

The Italian authorities took Zaghba's phone and passport but they were returned to him as there was insufficient evidence to accuse him of any terror-related offence. Of late, he had been living in east London, where his accomplices also lived.

There has been no official comment about the disclosures from UK authorities but Scotland Yard said Zaghba was not a police or MI5 "subject of interest".

The alleged ringleader of the London terror cell, Khuram Butt, 27, a British national who was born in Pakistan, was known to the Met police and MI5 for Islamist extremism. He was investigated by officers in 2015 but they found no evidence that he was planning an attack.

Last year, the father-of-two was featured in a Channel 4 documentary, The Jihadis Next Door, and was seen among a group of men unfurling a black Islamist flag in a London park.

Khalid Masood, 52, the terrorist who carried out an attack in March, mowing down pedestrians on Westminster Bridge in a car and stabbing to death a police officer outside the House of Commons, was known to the security services; he was a “peripheral figure” in an investigation some years ago.

And finally Salman Abedi, 22, the Manchester Arena bomber, was on the terrorism database because of his links to extremists in the city and his travels to Libya, where his parents came from.

Speaking about gaps in the MI5 net, Lord Carlile, a former counter-terror laws watchdog, said: "I feel a sense of disappointment this morning that the perpetrator Butt slipped off the radar. We need to review what happened in his case and learn the lessons so that the methodology of the response to known suspicions is improved."

Boris Johnson acknowledged the security services faced serious questions.

The Foreign Secretary said: "People are going to look at the front pages today and they are going to say: 'How on earth could we have let this guy or possibly more through the net? What happened? How can he possibly be on a Channel 4 programme and then committing atrocities like this?'

"And that is a question that will need to be answered by MI5, by the police, as the investigation goes on."

Intensive inquiries are now being mounted to establish how the three men knew each other with the police appealing for anyone with information to contact them.

Authorities say the current terror threat is unprecedented with 500 active investigations, involving 3,000 individuals in addition to 20,000 former subjects of interest.

Eighteen plots have been stopped since 2013, including five since the Westminster attack.

Chris Phillips, a former head of the UK National Counter Terrorism Security Office, told the BBC that the police were "spinning lots of plates" and noted how the terrorists could have decided "two hours before the attack happened to do this”. He asked: “What chance would the police have had then of stopping it?"

Zaghba, Butt and Rachid Redouane, 30, who claimed to be Moroccan-Libyan and is believed to have had a Scottish wife, killed seven and injured dozens when they launched their murderous rampage at London Bridge late on Saturday night.

It ended when armed police shot them dead just eight minutes after the first emergency call.

So-called Islamic State has claimed the rampage, which sparked fears that Britain is in the grip of a spate of copycat incidents.

In other developments:

*detectives arrested a 27-year-old man under the Terrorism Act at an address in Barking, east London, shortly after 8am on Tuesday;

*Australian nurse Kirsty Boden was the third victim killed in the attack to be named;

*Muslim women in east London have reported being subjected to physical and verbal abuse since the London Bridge atrocity;

*a British Transport Police officer told how he chased the three terrorists after his colleague was stabbed in the eye and

*15 people hurt in the incident remained in a critical condition.

On the eve of the election, the Prime Minister urged people to use their vote as an act of defiance against the terrorists.

She said: “What is important is for us on Thursday to show that our democracy has not been deterred, has not be damaged by these attacks and for people to go out there and exercise their right to vote."