Thomas Mair (left), 52, appeared at Westminster Magistrates' Court in London charged with murder and grievous bodily harm. He has also been charged with possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon.
Mair, who was wearing a grey, police issue tracksuit, refused to give his correct name and did not reply when asked to confirm his address and date of birth.
Asked at the court to confirm his name, Mair said: "My name is death to traitors, freedom for Britain." The judge then asked his lawyers to confirm his name, which they did.
Mair was remanded in custody and is due to appear at the Old Bailey on Monday.
Cox, a mother-of-two, was shot and stabbed to death in Birstall, West Yorkshire, on Thursday afternoon. The 41-year-old lawmaker was attending a regular constituency meeting when her assailant described as a far-right loner with mental health issues and neo-Nazi link struck.
A 77-year-old man, who came to the aid of Cox, was also injured and remains in a stable condition in hospital.
Vigils were held across the country last evening in memory of the Labour MP and Parliament will be recalled on Monday to allow MPs to pay further tributes.
British Prime Minister David Cameron and Opposition leader Jeremy Corbyn visited Cox's Batley and Spen constituency on Fridayin a rare joint appearance.
Meanwhile, the EU referendum campaigning has been suspended nationally until Sunday, with less than a week before polling day, after the fatal attack on Cox.
The Remain and Leave groups, which have not campaigned since Thursday, have cancelled events planned for Saturday.
"The campaign will continue to suspend all its planned national campaign events, street stalls, and rallies on Sunday as a mark of respect for the tragic murder of Jo Cox," Will Straw, executive director of pro-Remain Britain Stronger in Europe, said in a statement.
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