The mysterious man who used a rare narwhal tusk to charge at a knife-wielding terrorist on London Bridge has been revealed as a British civil servant.

Key points: Darryn Frost grabbed the narwhal tusk from the wall and used it to help subdue the terrorist

Darryn Frost grabbed the narwhal tusk from the wall and used it to help subdue the terrorist Usman Khan had served prison time for earlier terrorism offenses

Usman Khan had served prison time for earlier terrorism offenses Saskia Jones and Jack Merritt were killed in the attack

Darryn Frost told Britain's Press Association that he and others reacted instinctively when Usman Khan started stabbing people at a prison rehabilitation program that was being held at Fishmongers' Hall, next to the bridge, in November.

Mr Frost said he grabbed the narwhal tusk from the wall to help subdue Khan, even though the attacker claimed to be about to detonate a suicide vest.

The device turned out to be a fake but the intervention of bystanders, including Mr Frost, helped keep the death toll to two.

Video of the incident, which later spilled out onto the bridge, showed Mr Frost armed with the tusk and charging at Khan while other men used different weapons, including a fire extinguisher, to get him to the ground until police arrived.

Mr Frost said he was upstairs at the venue when the attack started, and he and a few others heard noise.

"I took a narwhal tusk from the wall and used it to defend myself and others from the attacker. Another man was holding the attacker at bay with a wooden chair," he said.

"He [Khan] had knives in both hands and, upon seeing me with the narwhal tusk, pointed at his midriff.

"He turned and spoke to me, then indicated he had an explosive device around his waist.

"At this point, the man next to me threw his chair at the attacker, who then started running towards him with knives raised above his head."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 4 seconds 1 m 4 s The stabbings occurred just before 2pm local time near London Bridge

The 38-year-old, along with others, later chased after Khan to the bridge where they helped restrain him.

The extremist, who had served prison time for earlier terrorism offenses, was shot dead on the bridge by police moments after he threatened again to detonate his vest.

There had been much speculation in the British media about the identity of the man who used the narwhal tusk in such a dangerous situation.

Mr Frost said he was "eternally grateful" to the former prisoners and the members of the public who were taking part in the meeting and ran to help.

"Not only do I want to thank those who confronted the attacker, but also those who put themselves in danger to tend to the injured, relying on us to protect them while they cared for others," he said.

Mr Frost said he was withholding many details out of respect for the victims and their families and because of the ongoing investigation.

He paid tribute to Saskia Jones and Jack Merritt, the two young people stabbed to death when the attack started.

"In reading about their lives and work I am convinced they represent all that is good in the world, and I will always feel the deep hurt of not being able to save them," he said.

Mr Frost praised those injured in the attack and said some had refused treatment until the more severely hurt were cared for.

"That consideration and kindness filled me with hope on that dark day," he said.

AP