British Prime Minister Boris Johnson's leadership of the Conservative party suffered a heavy double blow as voters rejected the Tories in two by-elections.

In the southwestern constituency of Tiverton and Honiton, the Liberal Democrats overturned a 24,000 Conservative majority to win, while the main opposition Labour Party reclaimed Wakefield in northern England.

The contests, triggered by the resignations of Conservatives hit by scandals, offered voters the chance to give their verdict on the prime minister just weeks after 41 per cent of his own MPs cast their ballots against him.

Losing both special elections will increase jitters among restive Conservatives who already worry the ebullient but erratic and divisive Johnson is no longer an electoral asset.