Bernard Enerio, 17, also told the jury that while John Mills was confronted by other gang members, he stepped forward and punched the 57-year-old businessman. "I just don't know why," he said in a barely audible voice.
But the youth, who had run away from home shortly before the attack last May, insisted he had not suspected he was taking part in a robbery.
Inner London Crown Court has heard how Mr Mills, whose wife is DPP Barbara Mills, was set upon by a six-strong gang after parking his Audi 80 car near his home in Albert Street, Regent's Park, London.
He was robbed of his wallet containing pounds 100, credit cards and photographs of his family.
Mr Mills told the jury he was left "dripping with blood" after the butterfly- style knife's six-inch long blade was thrust into his abdomen. He staggered the 40 yards to his front door and was only saved by emergency surgery to repair a severed artery.
He lost about 20 per cent of his blood before the bleeding was finally staunched.
Enerio, a Burger King employee, of Holmesdale Road, South Norwood, south- east London, denies causing grievous bodily harm with intent and robbery. He has admitted possessing a bladed weapon.
The trial resumes today.Reuse content