The economy grew 0.1% in the third quarter of 2025, the Office for National Statistics said, slowing from growth of 0.3% in the second quarter.
Economists polled by Reuters, as well as the Bank of England, had forecast 0.2% growth in gross domestic product for the July-September period.
In September alone, the economy contracted by 0.1%, against expectations for a flat reading.
Investors further ramped up bets that the Bank of England will cut interest rates next month, now seen as a roughly 82% chance from a 60% chance earlier this week, according to market pricing. The pound rose modestly against the dollar after the data.
The readings are unlikely to sway deliberations around Reeves' November 26 budget, with the economy still growing tepidly despite the government's intention to "kickstart" it.
A column chart with the title 'Latest UK gross domestic product (GDP) growth'
"The UK's unrelentingly sluggish growth trajectory is a headache for the Chancellor (finance minister) as it will inevitably mean a substantial fiscal shortfall at the budget, making major tax hikes look unavoidable," said Suren Thiru, economics director at the ICAEW trade body for chartered accountants.
The ONS reported a 28.6% collapse in the production of motor vehicles in September, the biggest such drop since April 2020 during the onset of the COVID-19 pandemic.
Overall, motor vehicles subtracted 0.17 percentage points from GDP growth in September alone and 0.06 percentage points for the third quarter as a whole.
The ONS cited a report from the SMMT motor industry trade body that described how "a cyber incident paused production at a major manufacturer".
JLR, owned by India's Tata Motors has three factories in Britain, which together produce about 1,000 cars per day.
The hack cost the British economy an estimated 1.9 billion pounds ($2.55 billion) and affected over 5,000 organizations, an independent cybersecurity body said in a report published last month.
A fall in commercial vehicle production in September added to the sector's woes.
The Bank of England expects to see the economy rebound in the fourth quarter with a 0.3% expansion.
In response to the figures, Reeves highlighted the fact that Britain had the fastest-growing economy among Group of Seven nations over the first half of 2025, but said there was more to do.