Britain's ruling Conservatives on Friday retained the former seat of ex-Prime Minister Boris Johnson but saw hefty majorities in two other constituencies blown away.

Rishi Sunak was expected to become the first prime minister in decades to lose three parliamentary seats on one day, but was spared humiliation due to a narrow victory in the northwest London seat of Uxbridge and South Ruislip.

The result, driven by opposition to Labour mayor Sadiq Khan's contentious expansion of a vehicle pollution tax to outer London, offered the embattled Tory leader some relief.

But the wiping out of his party's 19,000 majority in the Somerton and Frome seat and its 20,000 majority in the Selby and Ainsty constituency will come as hammer blows ahead of an expected general election in 2024.

"By-elections midterm for an incumbent government are always difficult, they rarely win them," Sunak told reporters Friday morning, striking a defiant tone while visiting Uxbridge and South Ruislip.

"The message I take away is that we've got to double down, stick to our plan and deliver for people... and earn people's trust for the next election."

But his Conservatives face defeat nationally next year if Thursday's results are repeated.