A rookie officer on his first callout to a live firearms incident fell as police confronted the London Bridge terrorists, firing through his legs while on the floor, an inquest has heard.

The City of London police officer, known as BX44, was in the armed response vehicle (ARV), which brought the three attackers’ 10-minute killing spree to a halt after they had murdered eight people on 3 June 2017.

Giving evidence anonymously at the Old Bailey on Thursday at the inquest into the deaths of the attackers, he told the court that as he got out at Stoney Street, Borough Market, he saw three men with large knives “closing us down”.

He said he fired first at the ringleader, Khuram Butt, 27, because he thought he was about to kill his colleague BX46, who was first out of the ARV.

“The red dot [from the gun’s sights] was on him but there was very little reaction and I was surprised he was still coming,” he said. “I continued to track him and fire shots until I had to break away to deal with Rachid Redouane, who at this point closed us down and was about to kill another colleague, BX45.

“The red dot was on him. I fired shots and there was no immediate reaction. I carried on firing until I had to deal with the third threat of Youssef Zaghba, who was on top of me. I was backing away trying to create a reactionary gap when I fired and fell backwards, and as I fell backwards I fired and from the floor I fired through my legs up to his chest.”

BX44 said he feared Zaghba was going to kill him and afterwards he kicked the attacker’s hand away from his chest as he assumed he was about to detonate an explosive belt.

Minutes after the first burst of gunfire he said he feared Butt was about to do the same as he saw his chest rise and fall. I did not see his hands but his arms started to move down slowly towards the suicide vest. “I thought he was going to detonate,” he said. “I fired shots. They were aimed shots.”

The armed officer said he thought Butt was dead, and was in the process of moving to the Wheatsheaf pub where people were taking shelter, but “noticed movement”, so fired more shots. BX44 fired 17 shots in all, the court heard.

The ARV driver, BX45, said: “As soon as I stopped the ARV, two males started running towards where I was getting out of the driver’s side. They were very close to me indeed, a matter of a metre or two. Both males had knives in their hands. I recall seeing one of them had what I believed was an IED [improvised explosive device] strapped to his chest.

“I shouted a verbal warning. It was a matter of a split second and the person was on top of me. I thought I was either going to be stabbed or his IED would explode. I shot the male. I continued shooting until the male hit the floor.”

BX46, who qualified as a firearms specialist in 2013, told jurors that when he came across Butt, wearing an Arsenal football shirt and carrying a knife, “I formed the view he was an immediate threat to me and I would have said words to the effect: ‘Armed police, stand still, drop the knife.’”

He said as he moved back to create a gap, Butt moved forward to within one or two metres and raised his knife. BX46 also believed the terrorist was wearing an IED.

“I already knew he had a knife and he was a threat to my life but now he was an even bigger threat, even with one or two metres a detonation would be fatal to colleagues, members of the public, anyone in the location,” he said. “So I aimed my rifle towards the male and I was moving back quickly and I moved the fire selector lever to fire and I pulled the trigger.”

BX46 said that later he noticed Butt lifting his head so “used lethal force to avert the danger of him detonating his device. I took an aimed shot to his head.”

The inquest continues.