A three-year-old British boy was crushed to death by a car passenger seat pushed back by his mother’s boyfriend, who complained he was too noisy.

Alfie Lamb cried out “Mummy” just seconds before the seat impacted him, the Old Bailey court was told.

The child had been placed in the rear footwell of an Audi A4 car by his mother Adrian Hoare, 23, before they started driving from their home in Croydon, southern England, last year, reports the BBC.

She and her boyfriend Stephen Waterson, 25, deny manslaughter.

Marcus Lamb, 22, had been driving the car with Mr Waterson in the front passenger seat, Miss Hoare behind him and Mr Lamb’s then-girlfriend, Emilie Williams, 19, in the other rear passenger seat.

Mr Lamb told the court when Alfie became noisy, Mr Waterson reacted angrily.

Alfie Lamb's mother Adrian Hoare and her boyfriend Stephen Waterson are on trial for manslaughter at the Old Bailey in London. (AP)

He then allegedly pushed his electric-powered seat backwards as Alfie cried: “Mummy”.

Prosecutor Duncan Atkinson said when others in the car realised the child was having breathing problems, Mr Waterson moved the seat forward.

"However, when Alfie made noise again, Waterson deliberately moved his seat back again, and kept it in that reversed position, squashing Alfie, as he again showed signs of breathing problems until he went ominously quiet," he said.

Alfie – described as a “smiley boy” – was suffocated and suffered brain damage.

Mr Atkinson said Ms Hoare had a duty of care to Alfie but did "nothing" to help him.

The court was told that after the incident, Mr Waterson and Ms Hoare had lied to police about what occurred.

Emergency services were called to Miss Hoare's home in Croydon on February 1 last year to a report of an unresponsive child.

Despite Mr Lamb’s attempts to resuscitate Alfie, it was obvious to police and medics that he had been dead for some time.

Ms Hoare allegedly told authorities Alfie had fallen asleep in a taxi, but was unresponsive when she tried to wake him.

But police found CCTV footage of Ms Hoare, Mr Waterson, Mr Lamb and Ms Williams in a dark Audi car.

Ms Hoare denies manslaughter, child cruelty, and common assault on Miss Williams.

Mr Waterson denies manslaughter, and intimidation of Mr Lamb.