The former soldier, who had battlefield medical training, held back tears as he relived the desperate attempts to save 48-year-old PC Keith Palmer’s life.
Giving evidence to the inquests into the victims’ deaths at the Old Bailey, Mr Ellwood said he performed CPR until an emergency doctor arrived at the scene.
When that medic decided the officer could not be saved, Mr Ellwood told him he would have to be ordered to stop giving help.
“I looked at him and said, ‘you’re going to have to tell me to stop. You must order me to stop. You’re going to have to make that decision’.
“He said, ‘Sir, you have done your best. We do need to stop’.”
The Tory MP for Bournemouth East also told how he ignored fears of a second terror attack as he tended to the PC.
“My brother was killed in a secondary attack in Bali (a 2002 terrorist bombing in Indonesia)… so I was very aware of that,” he said.
“I was concerned about what would happen if things were to ratchet up, but my immediate concern was that we had somebody who was clearly badly bleeding and needed assistance.”
PC Palmer was attacked as he tried to stop terrorist Khalid Masood from entering the Palace of Westminster on 22 March last year.
Masood had already ploughed an SUV into pedestrians on Westminster Bridge, killing Kurt Cochran, 54, Leslie Rhodes, 75, Aysha Frade, 44, and Andreea Cristea, 31.
Mr Ellwood described the “eerie silence” at the Palace of Westminster as it stayed in lockdown in the aftermath of the attack.
“It was very, very silent, it was a very strange end to a very traumatic four or five minutes, to suddenly be left completely alone with just one other person,” he said.
Both Mr Ellwood and the emergency doctor covered PC Palmer’s body and closed his eyes, he told the inquest, adding: “And I said, ‘I’m sorry’.”
Anthony Davis, who had a long career in the British Army and was trained in first aid, also stepped in to help, the inquest heard.
He said he told PC Palmer: “Come on son, stay with us”.
The officer was breathing at that point and had his eyes open but his pulse was weak, and he was bleeding heavily.
The former soldier described seeing “blood pooling in between the cobbles” as the extent of PC Palmer’s injuries became clear.
Recalling the moment that doctors decided they could do no more, Mr Davis said: “It was horrendous. A lot of people started to break down.”