Some of the 70-strong special forces unit – code-named “Blue Thunder” – were dispatched on the orders of the Home Secretary to join the hunt for the assailants.
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The unit has been rehearsing terror scenarios for more than a year after they were designated the country’s first line of defence against any attack on UK soil.
Named after its unmarked dark blue helicopter, the chopper was spotted in London Bridge shortly after three suspected terrorists ploughed into pedestrians with a van before knifing people in Borough Market.
The three attackers – who killed seven people and left 48 injured – were shot dead by armed officers within eight minutes of the first 999 call, police said.
Blue Thunder marksmen arriving on their Dauphin helicopter were deployed on the streets of London in case more attackers were at large.
Reports of the unit’s existence first emerged in March last year, with its soldiers rehearsing a variety of terror scenarios in preparation for a possible attack like those seen in France and Germany in recent years.
It works directly under the command of counter-terror police, operating alongside seven police liaison officers, and is based at three secret military airfields and two unnamed provincial airports, the Daily Express reported last year.
The unit – which can be deployed anywhere in the country – is also supported by an Apache helicopter gunship kitted out with powerful cameras to give commanders on the ground a live aerial video feed of unfolding events.
Saturday’s suspected terrorist incident comes just weeks after 22 people were killed during a suicide bombing at Manchester Arena.
Theresa May chaired an emergency Cobra meeting with ministers on Sunday morning in response to the latest incident.
She said outside Downing Street that it was time to say “enough is enough” when it comes to tackling terrorism.
“We cannot and must not pretend that things can continue as they are,” she said, adding that “difficult” and potentially “embarrassing” conversations were needed within the UK about dealing with the spread of extremism.
She said the London Bridge attacks were not directly connected with the bombing last month in Manchester or March’s attack in Westminster, which saw British-born terrorist Khalid Masood kill five people when he drove into pedestrians and launched a knife attack.
May said the country is “experiencing a new trend in the threat we face as terrorism breeds terrorism”.
An emergency ‘Blue Thunder’ response helicopter lands on London Bridge