LONDON (AFP) - The man accused of killing British lawmaker Jo Cox on a street in June repeatedly shouted "Britain first" as he shot and stabbed the mother-of-two, a court heard on the first day of his trial.

Prosecutor Richard Whittam told a London criminal court that Thomas Mair, 53, was guilty of the "cowardly attack" on Cox just days before Britain's vote on European Union membership.

Cox, a vocal supporter of the EU, had been due to meet her constituents in Birstall, northern England, on June 16 when she was attacked.

"As she arrived she was brutally murdered by one of her constituents, this defendant, Thomas Mair," he told the court.

"Jo Cox was shot three times and suffered multiple stab wounds. During the course of the murder Thomas Mair was heard by a number of witnesses to say repeatedly 'Britain first'." Whittam said the attack was premeditated "for a political and/or ideological cause." Mair, a gardener, appeared in court earlier in the day, confirming his name. The grey-haired and bearded Mair wore a black jacket, an AFP reporter in court said.

At his first court hearing in June, Mair had said his name was "Death to traitors, freedom for Britain".

The brutal street killing of Cox, who had defended immigration and refugee rights, shocked the nation and led to a three-day suspension in campaigning ahead of the EU referendum.

Mair denies Cox's murder, possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon.

The court, including Cox's parents and sister, also heard how a 77-year-old local man risked his life in an attempt to stop the attack.

The trial is expected to last for three weeks.