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Special forces were among the first on the scene of the attack , with eight SAS soldiers dropped in by easily maneuverable helicopters dubbed Blue Thunder.

The first four police “gunship” 4 x 4 vehicles to roar into action contained heavily-armed SAS men as it was feared a hostage situation in a packed bar was already unfolding.

More than a dozen undercover SAS men arrived in a fleet of vehicles from Scotland Yard and a secret London barracks, as part of the “immediate action” special forces unit.

Two unmarked Dauphin helicopters dropped off two teams of eight SAS specialists close to London Bridge, as Army bomb disposal experts also arrived by car.

A small number of Special Reconnaissance Regiment troops were also at the scene acting as “observers” searching for runaway terrorists from vantage points on rooftops.

The SRR would have also been used to call in extra troops or snipers if a hostage situation did unfold, directing helicopters to “blind-side” parts of rooftops where they could not be seen.

(Image: i-Images Picture Agency)

A military source said: “These helicopters are extremely agile and quick - with the ability to get into smaller spaces, travel in-between high buildings with relative silence and at speed, with eight soldiers.

“They are ideal for tackling a dynamic situation like this and if needed could have been used to bring in extra forces , snipers or hostage rescue rescue teams .”

The Dauphin choppers - which have been dubbed “Blue Thunder” craft - can also be used to take troops speedily and nimbly into battle, dropping troops on top of rooftops.

(Image: Twitter/@TheGSibold)

A number of specialist troops in “breaching” doors by blowing them up with explosives also arrived on the scene to prepare to smash their way through barricades.

Army bomb technicians are usually the specialists who blow in the doors with explosives as they have years of experience in executing the breach.

The director of special forces drove to his London office and remained in constant touch with an SAS officer who was on the ground.

(Image: PA)

According to a military source who was there on Saturday night the first assessment focused on a potential hostage nightmare and additional forces were called out.

The source added : “It was a very confused picture but when the teams arrived they were able to very quickly identify the three targets.

“But there was concern at least one person, the driver of the van, had fled. But this proved to be a false alarm.”

UK Special Forces have been on long-term attachment to police Counter Terror Command.

They are armed with the same Sig Saur rifle.

Some teams carry a modified Colt Commando rifle.

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(Image: Twitter/jo55adams)

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They are deployed on patrol with CTC officers, operate undercover at railway stations and major venues, posing as tourists and use motorbikes to patrol areas where police vehicle are easily spotted.

It is unclear if SAS men shot any of the attackers.

Just three weeks ago an entire SAS squadron took part in the biggest Special Forces counter terrorist training mission in recent years involving a hostage rescue and marauding knife attack.

During that training rehearsal four unmarked Army Air Corps Dauphin helicopters were used to ferry teams of SAS soldiers into hostage rescues situations – with pilots forced to land in difficult areas.