• U.K. leader Rishi Sunak's Conservative Party suffered significant losses at a flurry of local election results overnight.
• The votes are likely to be the last major political test for Sunak's party ahead of a general election.
• A date for the vote has not yet been set, although the prime minister said in January that his "working assumption" was it would take place in the second half of this year.

LONDON — U.K. leader Rishi Sunak's Conservative Party suffered significant losses at a flurry of local elections overnight, in a closely-watched indicator of sentiment ahead of the nationwide vote expected later this year.

The opposition Labour Party won a Blackpool South by-election, taking the seat from the Conservatives. New Labour MP Chris Webb beat his Conservative rival David Jones with a significant swing of 26%. The Reform UK party, founded by Brexit proponent Nigel Farage, came in third.

The northern town's by-election was called in the wake of former Conservative MP Scott Benton's suspension from the party after he was caught offering to lobby on behalf of gambling investors for money.

Labour leader Keir Starmer posted on the social media platform X that the "swing towards Labour in Blackpool South is historic."

"Britain is demanding change. It's time for a general election," he added.

Angela Richardson, deputy chair of the Conservative Party, told the BBC the result "was not unexpected," especially given the circumstances that sparked the by-election.

At smaller local elections across England, Labour also made sizeable gains. It took control of four councils — Hartlepool, Thurrock, Redditch and Rushmoor — while the Conservatives lost three. More council results are due.

The votes are likely to be the last major political test for Sunak's party ahead of a general election. A date for the vote has not yet been set, although it has to take place before Jan. 28, 2025 and the prime minister said in January that his "working assumption" was it would take place in the second half of this year.

Thursday's by-election is the 11th the Conservative Party has lost since the 2019 general elections.

Philip Rycroft, the former permanent secretary for exiting the European Union, told CNBC Friday that the results were a blow for Prime Minister Rishi Sunak.

In Blackpool, "the Conservatives narrowly missed being beaten into third place by Reform, so they were pretty much neck and neck with the Reform party in that by-election," he said. "So altogether a tough night for the Conservatives."