George Osborne, the Chancellor, scrapped a pro-EU speech he was due to deliver at Mansion House in the City, and instead wrote a new one in tribute to the murdered MP and stressing the importance of democratic freedom. “People are free in this country to live their lives as they choose and express themselves without fear,” he said.

“Society will protect their right to do so and hold to account those who disregard our laws. Today’s horrible events are an assault on all of these values.”

Mrs Cox was the first MP to be murdered since the death of Ian Gow, who was assassinated by the IRA in a car bomb outside his home in 1990. She is the first female MP to be murdered and is also believed to be the first killed by a constituent in modern times. Following the attack, MPs were told by the Commons’ authorities to call police if they have any concerns about security.

Police were poised to put extra security in place for Mrs Cox at her constituency surgery following a stream of harassing messages over three months, according to reports on Thursday night. There is no known link between the messages and Thursday's attack.

The authorities said they were reissuing guidance to MPs, who are back in their constituencies after Parliament broke up for the EU referendum recess on Wednesday. On Thursday night Britain First said that Tommy Mair had never had any contact with their group and condemned the killing.

Mr Mair’s house remained sealed off by police as forensic officers worked in the garden. Neighbours said he had lived there for more than 30 years, and on his own for the past two decades since the deaths of his mother and grandmother. They did not think he had a job or drove a car and described how he would do gardening chores for local people.