Under-fire British Prime Minister Boris Johnson yesterday faced a crucial test of his leadership as polls opened in a by-election in a constituency his party has never lost, where defeat would intensify calls for a new leader.

Johnson, 57, is already reeling after roughly 100 of his own MPs on Tuesday voted against government plans to introduce vaccine passes for large events. The UK leader's authority has been clobbered repeatedly in recent weeks by claims of corruption and reports that he and his staff broke coronavirus restrictions last Christmas.

Weeks of bad headlines have turned what would normally be a routine victory in a safe rural seat into a much more fraught process for Johnson's Conservatives. Ahead of polls opening in North Shropshire at 0700 GMT, Johnson was struggling to convince many to stick with him, prompting predictions of a historic loss in a seat where the previous Conservative MP won a huge 23,000 majority in the last election.

Voting, which closes at 2200 GMT with a result expected hours later, is increasingly seen as a referendum on Johnson's premiership just two years after his landslide general election victory in December 2019.

Defeat would probably see more MPs filing letters of no-confidence in their leader, which could trigger an internal party vote to remove him.

The same process saw his predecessor Theresa May ousted in mid-2019 after MPs including Johnson voted against her Brexit deal in parliament.

The Liberal Democrats have the best chance of overturning the Conservatives' huge majority, helped by supporters of the main national opposition Labour party lending them their votes.

The atmosphere is a far cry from May, when the Conservatives swept to an unprecedented by-election victory in the northeast England seat of Hartlepool on the back of a successful vaccine rollout.