An alleged killer went “crazy” and repeatedly stamped on a fellow inmate at Pentonville Prison as he lay mortally injured on the floor, a court heard.

Jamal Mahmoud, 21, was fatally stabbed in a row over contraband which was smuggled on to G Wing at the Caledonian Road jail, the Old Bailey was told.

Supervising officer Dizzy Vergo had allegedly been warned of trouble brewing but let the three alleged killers – Basana Kimbembi, 35, Joshua Ratner, 27, and Robert Butler, 31 – out of their cells.

Prison officer James Day said he spoke to Ms Vergo before the attack on October 18 last year and witnessed the violence as it unfolded.

He told jurors yesterday: “I was just at the bottom of the stairs on G2 (second floor of G Wing). It was just stamping of feet and loads of noise. You know something is going on. It’s what you hear when there is a fight happening.

“I looked up towards the 4s and 5s landing staircase, saw there was a lot of commotion going on, so I ran up the stairs to the 4s.”

Other officers were helping Mahmoud as he lay face down on the fourth floor, jurors were told.

Mr Day described a “melee” of prisoners crowding on to the landing, carrying weapons, including table and chair legs.

When Mahmoud was turned on to his back, the officer said blood from a stab wound was clearly visible.

Out of the corner of his eye, Mr Day saw Kimbembi running on the stairs.

He said: “As I turned round, he reached about the third step of the stairs and then jumped on Mr Mahmoud.

“Mr Kimbembi then leaned forward and held on to the railings and then proceeded to stamp on his head a further three or four times.”

The officer said the defendant had used “a lot of force” in the attack and then ran off “screaming and shouting” and acting like “he was crazy”.

The three defendants deny murder as well as wounding the victim’s associate, Mohammed Ali, with intent to cause him grievous bodily harm.

Under cross-examination, Mr Day was quizzed over an account he gave prison authorities on November 2 last year.

In it, he described relaying to Ms Vergo a conversation with an inmate that cells 213 and 214, where the defendants were housed, were in debt to another prisoner and there would be “trouble” if they were let out.

The governor advised the wing supervisor not to open the two cells, the court heard.

Ms Vergo spoke to the defendants and they told her they knew nothing about it, according to the account.

She asked Mr Day to call the governor for more advice and he reported back that he could not get through.

She then allowed the defendants out of their cells, the court heard. Michael Holland QC suggested that asking prisoners about a £10,000 contraband debt “might not get a truthful answer”. Mr Day agreed.

The trial was adjourned until Monday afternoon when Ms Vergo will give evidence.