A general view of the Canary Wharf financial district in London, Britain. Reuters

Gross domestic product shrank 0.3 percent in the fourth quarter of 2023 after contracting 0.1 percent in the prior three months, the Office for National Statistics (ONS) said in a statement.

That places the economy in recession, which is defined as two quarters of falling GDP in a row.

While economists predicted that the recession could be short-lived, the data is a big setback for Sunak, who has placed economic growth as a key priority.

It comes as the Conservatives badly trail the main opposition Labour party in polls.

Ahead of the general election, voters take part in two by-elections on Thursday, with the Conservatives fearful of losing one-time strongholds in Wellingborough, central England, and Kingswood in the southwest.

"The news that the UK slipped into technical recession in 2023, will be a blow for the prime minister on a day when he faces the prospect of losing two by-elections," said Capital Economics analyst Ruth Gregory.

"But this recession is as mild as they come and timely indicators suggest it is already nearing an end."

Agence France-Presse