Two of three terrorists named, one of whom was already known to police and security services

Questions will be raised about the effectiveness of British intelligence after it emerged that Butt, 27, from Barking in east London, who is believed to be the leader of the terror cell, was known to the police and MI5.

Scotland Yard stressed that there was no intelligence to suggest the attack was being planned.

At any one time MI5 has around 500 live investigations, of which several are plots to attack in the UK. Intelligence officers have identified 23,000 jihadist extremists living in Britain as potential terrorist attackers.

Cressida Dick, the Metropolitan Police Commissioner, said Britain was being hit by an "unprecedented" wave of terrorism and that it faced a “new reality” of copycat terror attacks. Theresa May said it was a case of "terrorism breeding terrorism".

It emerged that football fan Butt, who worked at KFC and Transport for London, wore an Arsenal shirt under his fake suicide vest during his murderous rampage.

Last year, he was filmed for the Channel 4 documentary, The Jihadi Next Door, unfurling the black flag of so-called Islamic State in London’s Regent’s Park and was twice reported to the authorities.

Butt was also caught on camera alongside two notorious preachers, who were well-known to the police and intelligence officials because of their extremist views. He is believed to have had links to the jailed preacher Anjem Choudary.

A friend of Butt called the anti-terror hotline because the killer became obsessed with watching hate preacher videos on YouTube. The police were later warned he was trying to radicalise children in a local park.

Nicknamed Abz, it was suggested Butt, a father-of-two, was so extreme in his beliefs that he:

*condemned as non-believers Muslims, who did not wear beards;

*would not speak to women directly:

*berated an Imam for encouraging worshippers to vote in the 2015 General Election and

*was banned from a local mosque after denouncing worshippers for being “un-Islamic”.

Last week, it is believed Butt invited neighbours to a barbecue meant to be a “send-off” ahead of Saturday night’s attack.

Mark Rowley, the Assistant Commissioner, explained how an investigation into Butt began in 2015. “However,” he explained, “there was no intelligence to suggest that this attack was being planned and the investigation had been prioritised accordingly."

He added: "Work is ongoing to understand more about them, their connections and whether they were assisted or supported by anyone else."

Redouane, 30, also from Barking, is believed to have been a pastry chef, was married to Scottish woman and who at one time lived in Ireland.

Police chiefs in Dublin said they were liaising with their UK counterparts amid reports that the killer had Irish papers and had been living in the Dublin suburb of Rathmines until as recently as one year ago.

It is believed Redouance had not been brought to the attention of the security services but also used an alias, Rachid Elkhdar, claiming to be six years younger than he was.

Among those arrested in London on Sunday were Charisse O’Leary, 38, the mother of Redouane’s 18-month-old daughter. She was heard pleading “don’t shoot, don’t shoot” as officers stormed the property in Barking.

In other developments:

*11 people arrested on Sunday remained in custody;

*two further addresses in east London were searched on Monday morning with a number of people detained;

*the first victim of the attack was officially named as Canadian national Christine Archibald, 30, who died in her fiancé’s arms and

*Jeremy Corbyn called on Theresa May to resign over police cuts but the Prime Minister insisted the Metropolitan Police was "well-resourced" and had "very powerful counter-terrorism capabilities".

Other victims are thought to include James McMullan, 32, from Hackney - his sister said police had told her his bank card was found on a body at the scene of the attack - and an unnamed French citizen, confirmed dead by Jean-Yves Le Drian, France’s foreign affairs minister.

Meantime, five people, reported missing, have not been seen since the attack: Sebastien Belanger; Xavier Thomas and his girlfriend, Christine Delcros; Australian Sara Zelenak and Spaniard Ignacio Echeverria, 39.

On Monday evening at a vigil for the dead and injured in Potters Fields Park, just a short walk from the scene of Saturday night’s bloodshed, Sadiq Khan, the London Mayor, warned extremists they would never win after mourners observed a minute's silence.

To huge applause, Mr Khan said: "As a proud and patriotic British Muslim I say this: you do not commit these disgusting acts in my name; your perverse ideology has nothing to do with the true values of Islam. You will never succeed in dividing our city...London will never be broken by terrorism.”

In the latest outrage, three terrorists killed seven victims and injured dozens more in the London Bridge area just after 10pm on Saturday.

Pedestrians were struck by a van on London Bridge before the attackers stabbed a police officer and revellers around nearby Borough Market with 12-inch knives.

Hundreds cowered in pubs and restaurants, barricading themselves inside as the attackers stalked the street. As the murderous rampage unfolded, one of the terrorists shouted: “His is for Allah.”

The terrorists, who were wearing fake suicide bomb vests, were shot dead by eight officers after police opened fire with a hail of 50 bullets.

Tales of heroism emerged, with one British Transport Police officer taking on the attackers armed only with his baton before being stabbed in the head, face and leg. Three other officers were hurt in the attack.

Of the 48 people injured, 36 are currently being cared for in hospitals across London with 18 remaining in a critical condition. Several are foreigners, including people from France, Australia and New Zealand.

The so-called Islamic State terror group has claimed responsibility for the London Bridge attack, which, after the rampage in Westminster in March, has sparked fears Britain is in the grip of a spate of copycat incidents.

The Prime Minister said: "You have had the well-planned terrorist attacks in the past and you have had the lone wolves. Now, we see this terrorism copying others, using crude means to attack us and do us harm."

She told the London Evening Standard that the "pace of intent to try to do us harm has been significant" and that the threat had become "more complex".

Mrs May also claimed that society had to be "more willing to call out terrorism and extremism" having been "too tolerant".

The PM was asked whether or not she would recognise those who helped in the Manchester and London attacks in the Birthday or Christmas honours lists but said: "We never talk about those things."

She added: "What I will say is that we are making sure that we identify acts where police and others have really given people help and support and shown bravery in this incident."