A policeman who allegedly stamped on an arrested teenager, leaving him struggling to breathe, was accused of trying to “smear” the youth as he denied the charges.

Prosecutors claim Pc Joseph Harrington, 29, assaulted the boy, then 15, while he was being held in custody in Forest Gate police station.

The Met officer told Southwark Crown Court yesterday that he simply restrained the boy after he became aggressive, and suggested the youngster had a history of making false allegations.

The court heard the teenager, who cannot be identified for legal reasons, had been arrested on August 11, 2011, and was taken from Stratford police station to Forest Gate police station.

He was in the caged area near the custody suite when Harrington walked in and made a reference to encountering the boy previously.

The 17-year-old said: “I laughed at him. I thought: ‘I’m already arrested for something I haven’t done; I’m handcuffed, in a police cage, what could get worse?”’

He told the court that officers dragged him across the floor before Harrington allegedly stamped on his back.

But Harrington denied using unnecessary force and insisted he was following his training.

Duncan Atkinson, prosecuting, suggested Harrington had “had enough” because he was exhausted from policing the capital during the riots, which had happened just days earlier.

But Harrington said he was “in fear” of being assaulted by the youth.

He said the teenager was known for assaulting police and told the court he believed the youth was also known to have made false allegations.

Mr Atkinson said: “Are you trying, in any way you can, to smear (him)?”

“No sir,” Harrington replied.

Harrington, of Walthamstow, denies assault.

The trial continues.