LONDON—The number of unemployed Britons has soared to the highest level for 17 years, increasing pressure on Prime Minister David Cameron to do more to fuel growth and raising fresh questions about the wisdom of his government's fierce austerity drive.

The jobless total rose 128,000 in the three months to October to 2.64 million, the highest level since the third quarter of 1994, the Office for National Statistics said Wednesday. The jobless rate rose to 8.3%, its highest level since the three months to January 1996.

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