One of the police marksmen who shot dead Jean Charles de Menezes told an inquest today that knowing he had killed an innocent man was something he would have to "live with for the rest of my life".

Giving evidence for the first time, the officer from Scotland Yard's elite CO19 specialist firearms unit, identified as C12, broke down as he recalled the moment he shot De Menezes, saying he was convinced at the time the Brazilian was a suicide bomber about to detonate his device.

Had De Menezes remained seated on the tube train when challenged by police rather than standing up and advancing he would not have been shot, the officer added.

De Menezes' mother, Maria Otone was in court as C12 – who offered his "sincere regret" to the family – described his feelings when he learned the next day that the man he had shot was not Hussain Osman, one of four suicide bombers who carried out a failed attack on London's transport network the previous day.

C12 told the inquest into the 27-year-old Brazilian's death that he felt "a sense of disbelief and of shock, sadness, confusion".

"Everything I have ever trained for - for threat assessment, seeing threats, perceiving threats and acting on threats - proved wrong," he said.

"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 marksman told how he came face to face with De Menezes inside a cramped tube train at Stockwell station in south London on July 22 2005.

He recalled that the electrician got up and walked towards him, and kept moving even after he shouted "armed police" and pointed his gun at him.

"He continued on his forward momentum towards me. 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 from happening."

De Menezes was shot seven times in the head at point-blank range.

Asked why he shot De Menezes three times, C12 replied: "I had to be certain that life was extinct, that there wasn't any more threat, that this person couldn't detonate a bomb.

"Our training is, or has been, that a critical shot is a single shot, probably to the base of the skull, which will cause instant paralysis. I was not in a position to get there to do that. I fired the number of shots because I detected movements, albeit might have been movement caused by the bullets."

The inquest was halted briefly after C12 became distressed and could not continue. When it resumed, he recalled being "covered in blood" after the shooting.

As he began recounting the moment the Brazilian was killed, C12 apologised further for any distress his account caused.

"I will try and be as delicate as I can for the benefit of the family, but there are some things that have to be said.

"I can't begin to put myself in the position that they are faced with. I am a family man myself and to lose a son or any member of your family in this situation - I just couldn't believe it. And I offer my sincere condolences, I really, really respectfully do that."

C12 spoke of his frustration at the lack of contact with his superiors as he prepared to intercept De Menezes. He said he could not understand a "radio silence" order from the Scotland Yard command centre as the Brazilian got off his bus at the station.

"We were very, very close to the tube station as the subject went in and I remember there being a radio silence and I was frustrated by this.

"If I could add that my sense of frustration was great to say the least.

"My thoughts then were 'He's entered the tube already' and I was just frustrated."

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