By Michael Holden

LONDON (Reuters) – British lawmaker Jo Cox, who was killed in the street shortly before the June 23 referendum on EU membership, died in a pre-meditated murder carried out for a political or ideological cause, jurors were told on Monday.

As the prosecution opened its case against Thomas Mair, 53, who is charged with Cox’s murder, the jury at London’s Old Bailey court were told witnesses had heard Mair repeatedly say “Britain First” during the attack.

Cox, a 41-year-old mother of two young children, was shot and repeatedly stabbed as she arrived for an advice session with constituents in the town of Birstall, part of her electoral district in northern England.

The murder of Cox, a former aid worker who had been an ardent supporter of staying in the EU, shocked Britain and led to the suspension for several days of referendum campaigning which had been growing increasingly bitter.

Mair is also charged with causing grievous bodily harm to 77-year-old Bernard Carter-Kenny, who tried to help Cox during the attack, and possession of a firearm and a dagger.

Prosecutor Richard Whittam told the jury 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.

“Thomas Mair’s intention was to kill her in what was a planned and pre-meditated murder for a political and/or ideological cause.”

“DEATH TO TRAITORS”

Carter-Kenny risked his own life and was stabbed with the same knife Mair used on Cox, Whittam added.

Earlier on Monday, with Cox’s mother, father and sister in court watching, eight men and four women were sworn in as jurors to hear the case which the judge Alan Wilkie said had attracted and would continue to attract considerable attention.

Mair, balding with a gray goatee beard and wearing a dark blue suit and black tie, sat silently in the dock flanked by three security guards.

At a hearing in October, he declined to respond when asked if he was guilty so the judge recorded not guilty pleas.

At the first court hearing following his arrest, Mair had said his name was “death to traitors, freedom for Britain” and the case, due to last two weeks, is being treated as a terrorism matter.

His lawyer has also previously told London’s Old Bailey central criminal court where the trial is being held that medical issues would not feature in the defense argument.

Cox’s murder briefly united politicians divided over the EU question and also led to questions about the security of lawmakers in their constituencies.

“As the trial starts I’d encourage everyone to remember Jo’s life and what she stood for, not the manner of her death,” her husband Brendan wrote on Twitter on Saturday.

The last British Member of Parliament to have been killed before Cox was Ian Gow, who died after an Irish Republican Army (IRA) bomb exploded under his car at his home in 1990.

(Reporting by Michael Holden; editing by Stephen Addison and Estelle Shirbon)