Sir David Amess, the MP for Southend West, is the ninth parliamentarian to be violently killed in office in British political history.
The 69-year-old father-of-five was stabbed to death during a constituency surgery at Belfairs Methodist Church in Leigh-on-Sea on Friday.
A 25-year-old man has been arrested on suspicion of murder, Essex Police said.
Tributes to Mr Amess quickly poured in from his parliamentary colleagues, with Northern Ireland secretary Brandon Lewis saying he was “one of life’s truly nice people, a gentleman, who was always ready to give his help to anyone who needed it”.
The killing comes five years after Labour MP and mother-of-two Jo Cox was assassinated at the age of 41 by a far-right terrorist, who shot and stabbed her outside her North Yorkshire surgery.
Jo’s widower, Brendan Cox, described Mr Amess’s killing as “an attack on democracy itself”.
“There is no excuse, no justification. It is as cowardly as it gets,” he added.
Before the death of Ms Cox, the last MP to be killed in office was Ian Gow, 53, who lost his life after the IRA detonated a car bomb at his Sussex home in 1990.
The Eastbourne MP, who had previously served as a private parliamentary secretary to Margaret Thatcher, was not the only British politician to die at the hands of the terrorist group.
Sir Anthony Berry was killed by the organisation’s bombing of Brighton’s Grand Hotel during the 1984 Tory Party conference, three years after the group shot dead the Ulster Unionist Party MP Robert Bradford in Belfast.
In 1979, the Conservative MP and former Northern Ireland secretary Airey Neave was assassinated by the Irish National Liberation Army. His car was blown up as he drove it out of the parliamentary car park.
These are the only sitting MPs to have been murdered since World War II. However, Andrew Pennington, who had previously served as a politician, was killed when he rushed to defend his boss, the Liberal Democrat MP Nigel Jones, from a samurai sword attack in January 2000.
Spencer Perceval, who was shot in the Commons in 1812, is the only prime minister to have been assassinated in office.
Additional reporting by PA