Demand for new cars fell 6.7% last month, the automotive industry said.
Some 10,348 fewer cars were registered in October than during the same month in 2018, according to the Society of Motor Manufacturers and Traders (SMMT).
The figure reflects a tough environment for businesses and consumers as economic and political uncertainty continue to hit confidence, the trade association said.
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The decline was driven by a 13.2% drop in demand from private consumers.
Sales of diesel models were down 28.3%, while petrol cars fell by 3.2%.
Alternatively fuelled cars such as hybrids and battery electrics took a market share of 9.9%, which is the highest on record.
Year to date figures show the new car market declined 2.9% during the first 10 months of the year compared with 2018.
O2's UK performance was a silver lining in a disappointing few months for its Spanish parent.
Spain's Telefonica said its operating income before depreciation and amortisation (oibda) plummeted nearly 32% in the three months to the end of September. Analysts had been expecting a drop, but not by that much.
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However, in the UK, where the company operates under the O2 brand, the same measure grew 5.7% to £489 million, as the business attracted 1.9 million new customers, a 5.6% increase. Revenue also grew in theiscountry.
"As the UK's largest network, we're proud of our ability to keep attracting and retaining customers, which ensures a healthy business and drives both top and bottom-line growth," said UK chief Mark Evans.
He said the introduction of limitless data plans and O2's coverage had helped drive new customers into its arms.
Telefonica's results were dragged lower by poor performances in its home market, Spain, despite strong showings in the UK and Brazil.
Separately, the telecoms provider said it was partnering on a project using next-generation 5G technology to run driverless cars in London.
It reiterated its predictions for the fourth quarter, but shares still fell 2.2% to €6.78 euros.
Weir has warned over profits in its oil and gas division and revealed a fifth of its US workforce has been axed as it battles against tough trading in America.
The Glasgow-based group said it had cut around 450 US jobs under plans to slash annual costs by £30 million across the oil and gas division.
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It suffered a 32% plunge in gas and oil orders year-on-year in the third quarter after US customers cut back operations and ordered less machinery.
This left the division only "moderately profitable" in the three months to September 30 and Weir said the fourth quarter would be "sequentially lower".
Overall orders in the third quarter rose 4% and were flat year-on-year as its recent record £100 million Australian mining contract helped offset the oil and gas woes.
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