LONDON — The British economy unexpectedly shrank during January, piling pressure on Treasury chief Rachel Reeves before a key statement about the state of the public finances later this month.

Official figures Friday showed that the economy, the world’s sixth-largest, saw output drop by 0.1% during the month, in contrast to expectations for a modest increase and December’s solid 0.4% gain.

The Office of National Statistics said the weak performance was largely due to bad weather affecting the manufacturing and construction sectors, despite a solid performance by the services sector, which accounts for around 80% of the British economy.

Though monthly figures are notoriously volatile and prone to future revision, the decline highlights the struggles the Labour government, elected last July after 14 years in opposition, is having in generating growth.

Prime Minister Keir Starmer has said raising the U.K.’s economic growth is his government’s top priority over the next five years. Since the global financial crisis in 2008-09, the British economy’s growth performance has been historically lackluster.

Starmer has pledged to boost living standards and generate funds for cash-starved public services.

Critics say Treasury chief Reeves has been partly responsible for the economic slowdown since Labour returned to power, because she was overly gloomy when taking on her role and increased taxes, particularly on businesses.

After the January figures, she vowed to “turn around the poor performance of more than a decade” and that the government will go “further and faster” in promoting policies to boost growth.