The Conservative Party will not contest the by-election in Batley and Spen resulting from the death of Labour MP Jo Cox, Downing Street has said.
No date has yet been set for the vote to find a new MP for the West Yorkshire seat, which Ms Cox had held for just over a year.
She entered Parliament at last year's general election, holding the seat for Labour by a comfortable majority of 6,057.
The constituency has been held by Labour since 1997, but was previously a Conservative seat since its creation in 1983.
The mother of two died after being shot and stabbed in the street outside her constituency advice surgery at lunchtime on Thursday.
Tommy Mair, 52, was detained by police over the attack near his home in Birstall, West Yorkshire.