A police marksman who shot a man wrongly suspected of being a terrorist after the 7/7 London bombings said he was certain “we were going to die” if he did not act.

Jean Charles de Menezes was killed a fortnight after suicide bombers exploded devices on three Tube lines and a London bus on July 7 2005, killing 52 people.

The firearms officer, who has remained anonymous for nearly 20 years, has spoken publicly about his actions for the first time as part of Channel 4’s Shoot To Kill: Terror On The Tube documentary.

He said Mr de Menezes’ demeanour having been identified as a suspect led him to believe the Brazilian national was about to detonate a bomb.

“Reliving it in this detail is painful,” the officer said.

“I want to make sure that people understand these decisions, although they’re taken quickly, they’re not taken lightly.

“Because of his actions, what he did, the information we received, it left me with no other conclusion than I had to act or we were going to die.”

Would-be suicide bombers targeted the transport network again on July 21, but their devices failed to explode.

The following day Mr de Menezes, a 27-year-old electrician, was mistaken for one of the suspects because they were linked to the same block of flats.

He was shot seven times by two marksmen at Stockwell Tube station, south London, after officers followed him onto the train carriage.

The firearms officer, known only as C12, said the way Mr De Menezes stood up had “triggered” something in his head.

“He knew who we were. He still continued on his forward momentum as I had my weapon up, pointing at his head,” he recalled.

“I remember the surveillance officer then in full body contact with him, and apparently what he was trying to do was pin his hands so that he couldn’t detonate.

“I’m expecting an explosion at any moment, he’s gonna blow. We’re gonna die. But that’s the nub of it.

“If I don’t do something now, we are all going to die.”

On July 23, Scotland Yard confirmed Mr de Menezes was not connected to the July 21 attacks.

Dame Cressida Dick, who was promoted to Metropolitan Police commissioner in 2017, led the operation in which Mr de Menezes died.

A jury cleared her of any blame in his death at the end of the prosecution of the Met under health and safety laws.

Sir Tony Blair, who was prime minister at the time, recalled he was in a bilateral meeting with the Chinese president when he received news of the 7/7 bombings.

“I got handed a piece of paper saying there’d been a series of incidents on the London Underground,” he said.

“At that time I didn’t have any more information about it. The moment you get passed a note like that, you expect the worst.”

Shoot To Kill: Terror On The Tube will air on November 10 and 11 on Channel 4.