Opening his Old Bailey trial on Monday, prosecutor Richard Whittam QC told jurors how the mother-of-two supported the Remain campaign leading up to the referendum on 23 June.
“As she arrived she was brutally murdered by one of her constituents, this defendant, Thomas Mair,” he told jurors.
“It was a cowardly attack by a man armed with a firearm and a knife. 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’.”
Thomas Mair appeared with grey hair and a light beard PA
The court heard Mr Mair also stabbed 77-year-old Bernard Carter-Kenny while the latter was risking his life for Ms Cox, before paramedics attempted an emergency thoracotomy.
Mr Whittam added: “The murder took place whilst she was performing her role as a Member of Parliament. Thomas Mair’s intention was to kill her in what was a planned and premeditated murder for a political and/or ideological cause.”
He later said Mr Mair, in the run up to the killing, searched the web for Nazi material, the Ku Klux Klan, murdered Tory MP Ian Gow and Remain-supporting MP William Hague.
The prosecutor also said Ms Cox, who was proud of being “made in Yorkshire”, “achieved many things and significantly she believed in the open exchange of ideas and advocated toleration and equal rights”.
Mr Mair, who allegedly used a .22 gun adapted to kill and a dagger-like knife, denies murder, possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon.
Mr Mair, from Birstall, also pleaded not guilty to causing grievous bodily harm with intent to Mr Carter-Kenny.
Northern Ireland’s Ian Gow was the last MP before Mrs Cox to be killed, when an IRA bomb exploded under his car at his house in 1990.