The Liberal Democrats enjoyed their biggest swing in two decades in a surprise upset at the Witney by-election, leapfrogging Labour and Ukip to take second place.

The result in David Cameron’s former seat was still a comfortable win for the Conservative candidate, Robert Courts, a barrister and local councillor, but he won fewer than half of the 35,201 votes chalked up by the former prime minister in last year’s general election.

After a short speech, in which he thanked his wife and family and praised his “brilliant” predecessor, Courts left the count without speaking to the press.

Home Office minister Brandon Lewis said the party considered it a good result. “This is almost exactly the same as David Cameron got when he first stood. We’ve seen Labour crashing, Ukip crashing and the Lib Dems benefiting from that, but the reality is Robert’s got a very good majority and a clear win for the Conservatives.” He denied it was unusual for Courts to have left the count abruptly.

But with the Conservatives down by 15.1 percentage points, and the Lib Dems up by 23.5, the result represented a 19.3% swing from the Tories to their former coalition partners.

The party claimed such a swing at a general election would snare an extra 26 seats from the Conservatives nationwide.

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