Mike Hill, the Labour MP for Hartlepool, has resigned his seat in the House of Commons.
It will trigger a by-election in the constituency, which Labour won at the 2019 election with a majority of 3,595 over the Conservatives.
A Labour Party spokesperson said Hill’s resignation was “with immediate effect”.
The 57-year-old had represented the north east of England town since 2017.
The vote is likely to be timed for May 6, the same day as local elections. It will be seen as a key test for Keir Starmer, who will have been Labour leader for one year in April.
Polls have highlighted the struggle Starmer is facing, with national voting intentions consistently putting the Conservatives ahead of Labour.
The UK has one of its largest ever sets of elections in May. As well as county, district and city elections, there will be votes for eight London and metro mayors, and for the Scottish and Welsh parliaments.
It comes as polling analysis suggested Boris Johnson has secured a “vaccine bounce” in popularity, largely driven the over-55s who have been given the Covid jab.