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British spooks stood by and watched as the ­ London Bridge ­killers loaded their van and set off to launch their atrocity.

Ringleader Khuram Butt, a former disciple of jailed hate preacher Anjem Choudary, had been on MI5’s radar since mid-2015 as a “subject of interest”.

It was thought he was likely to be planning an attack – and up to 30 intelligence staff and undercover police were watching him around the clock.

But the surveillance operation was eventually downgraded as MI5 became more concerned with other suspects and assessed Butt as being more likely to travel to Syria to join Islamic State .

And it has emerged that when he and accomplices Rachid Redouane and Youssef Zaghba were seen loading a van outside his flat in Barking, East London, before last June’s atrocity, intelligence officers did not bother calling police.

(Image: PA)

Two hours later, the vehicle was used to mow down pedestrians before the three jihadis went on a knife spree, ­stabbing people at pubs and restaurants.

A report by David Anderson QC, the Government’s former independent reviewer of terrorism legislation, confirmed Butt had been under surveillance by MI6 agents and Scotland Yard, and he was still the subject of “a live investigation” on the night of the attack.

James Hodder, whose girlfriend Kirsty Boden was among the victims, said he was shocked to learn that the murderous rampage could have been prevented.

(Image: Getty)

Nurse Kirsty, 28, was killed while running into danger to try to help others and has been hailed as the Angel of London Bridge.

James said: “It’s important the ­authorities know that society, including the victims’ families, will hold them accountable for any errors.”

The attackers ploughed their white rental van into pedestrians on the bridge before it crashed into railings and they abandoned it.

The terrorists then began ­stabbing victims outside Borough Market with 12in ceramic knives.

(Image: PA)

Butt, 27, Redouane, 30, and Zaghba, 22, had taken large amounts of steroids before the massacre, a court heard this month.

The horror lasted from 10.07pm to 10.16pm – when the attackers, who killed eight people, were shot dead by police.

­Canadian Christine Archibald, 30, and Frenchman Xavier Thomas, 45, died in the van attack.

Along with Kirsty, Alexandre Pigeard, 26, Sara Zelenak, 21, Sebastien Belanger, 36, James McMullan, 32, and Ignacio Echeverria, 39, died in the knife attack.

(Image: PA)

Records of the van’s movements on the evening indicate that the trio had scouted Trafalgar Square and Oxford Circus as potential targets.

However, the heightened security in those areas meant they finally settled

on launching their attack on London Bridge.

Inquests into the horror will take place next year at the Old Bailey.

The disclosure about the monitoring of Butt follows claims that security services also missed opportunities to prevent the suicide bombing at Manchester Arena last May.

A homemade bomb detonated by Islamist Salman Abedi , 22, killed 23 people and injured more than 500.