The police marksman who killed Jean Charles de Menezes described in chilling detail yesterday how he became convinced that the Brazilian was about to detonate a bomb and believed he had no choice but to shoot him in the head.

Facing the mother of Mr de Menezes and other family members across the courtroom, the specialist firearms officer broke down as he recalled firing at point-blank range after Mr de Menezes was wrestled into a seat inside a busy Tube train.

The officer from Scotland Yard’s CO19 specialist firearms unit, identified only as C12, told the inquest how the Brazilian had at first got up and walked towards him – and kept moving even after he shouted “armed police” and pointed his gun at him.

“It was at that stage that I just formed the opinion that he’s going to detonate, he’s going to kill us and I have to act now in order to stop this happening.”

C12 choked back tears as he described how he shot Mr de Menezes three times. He also spoke of his “great frustration” with police commanders when instructions dried up as the electrician reached Stockwell Tube station on July 22, 2005. Reception on his Cougar radio was “awful” as he tried to listen in on communication between the surveillance officers following Mr de Menezes.

Speaking of the events for the first time in public, C12 told the inquest of his shock at later learning he had killed an innocent man. “Everything I have ever trained for – for threat assessment, seeing threats, perceiving threats and acting on threats – proved wrong.

“And I am responsible for the death of an innocent man. That is something I have got to live with for the rest of my life.”

The highly experienced officer recalled a scrum-like ruck as he pointed a gun towards Mr de Menezes’s head while a surveillance officer known only as Ivor pinned the Brazilian down. “I brought the handgun up from my leg and pointed it up to shoulder level,” he said. “He [Ivor] immediately grappled him into his seat. It just reconfirmed to me that I had to use force as soon as possible.”

Mr de Menezes, 27, was shot seven times in the head at Stockwell in South London after being mistaken for failed suicide bomber Hussain Osman.

Screens were brought into the courtroom at the Oval cricket ground in South London so the marksman could be seen only by the coroner, jury, lawyers and the Menezes family.

Watching in court were the Brazilian’s mother, Maria Otone de Menezes, 63, and brother, Giovani da Silva, 36. Afterwards Mrs de Menezes said that listening to the account of her son’s final moments was “very difficult”.

The inquest was told that C12, who has been a Metropolitan Police officer for 25 years, qualified as a specialist firearms officer in 1998 after undergoing a demanding selection and training process, including courses in abseiling into buildings and using shotguns. But despite being regularly deployed on firearms operations, he never needed to use his weapon against an alleged offender until the day he chased Mr de Menezes.

He and his team were told that they would have to detain any suspects who emerged from a block of flats in South London linked to failed suicide bomber Hussain Osman. The firearms officer said he was not initially told of the urgency of his deployment but it was made clear that he might have to tackle a dangerous terrorist. C12 spoke of his fears that he might not “go home again at the end of the day”.

He broke off from the evidence to tell Mr de Menezes’ family: “ I am a family man myself and to lose a son or any member of your family in this situation . . . I want to offer my sincere condolences. I really, really respectfully do that.”

The inquest was adjourned until Monday, when C12 will be cross-examined by Michael Mansfield, QC, for the de Menezes family.

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