Voters will head to the polls in two by-elections that could give an indication of the scale of the challenges facing the main political parties ahead of a national contest later this year.

Rishi Sunak is braced for tests in Wellingborough and Kingswood, where Labour hopes to flip Tory majorities in the tens of thousands.

Headlines this week have been dominated by a different by-election – the upcoming Rochdale vote, in which Labour’s candidate has had party support withdrawn over remarks he made about Israel and Jewish people.

But Thursday’s results will also be significant, with a Tory defeat in either constituency meaning that the Government has clocked up more by-election losses in a single parliament than any administration since the 1960s.

Both votes are seen largely as two-horse races between Labour and the Conservatives – though the Tories are also threatened by strengthening support for Reform UK, which is targeting disgruntled voters on the right.

The circumstances surrounding the by-elections could also prove difficult for the governing party.

Kingswood’s vote was triggered by Chris Skidmore’s resignation as an MP in protest at Government legislation to boost North Sea oil and gas drilling.

He won the Gloucestershire constituency for the Tories at the past four general elections, before which Labour held it at every general election since 1992.

The Opposition needs a much smaller swing to overturn the Conservative majority of 23% than the ones it recently secured in Tamworth, Selby and Ainsty and Mid Bedfordshire.

The by-election in Wellingborough comes after former Tory MP Peter Bone received a six-week suspension from the Commons when an inquiry found he had subjected a staff member to bullying and sexual misconduct.

He won the Northamptonshire constituency at every general election from 2005 to 2019, with Labour coming second in four of the five contests and Ukip in 2015. His majority in 2019 was 36%.

The swing needed by Labour to win the seat is at 17.9 percentage points – in other words, the equivalent of a net change of 18 in every 100 people who voted Tory in 2019 switching sides.

This is still a smaller swing than the ones managed by Labour in 2023 at the by-elections in Tamworth, Selby & Ainsty and Mid Bedfordshire.

According to the latest voting intention poll from Savanta, the lead enjoyed by Labour over the Conservatives has dropped by seven points after a turbulent couple of weeks for the party.

Sir Keir Starmer’s party has held a sustained double-digit advantage over the Tories in national opinion polls, but the past week has been overshadowed by criticism of remarks made by Rochdale candidate Azhar Ali.

Mr Ali apologised after he was recorded during a meeting of Lancashire Labour members suggesting that Israel had taken Hamas’ October 7 attack as a pretext to invade Gaza.

The party leadership initially stood by him, but withdrew its backing after the Daily Mail reported he had also blamed “people in the media from certain Jewish quarters” for the suspension of Labour MP Andy McDonald.

A second parliamentary candidate, Graham Jones, was suspended on Tuesday after audio obtained by website Guido Fawkes appeared to show the former Labour MP using abusive language at the same meeting Mr Ali attended.

Polls open at 7am and close at 10pm on Thursday, with the results expected to be declared in both constituencies some time after 4am.

The Rochdale by-election will take place separately on February 29.

Due to new laws brought in by the Government, voters will need to bring photo ID – such as a passport or driving licence – in order to cast their ballots.