Britain's Prime Minister Boris Johnson arrives at Healey's Cornish Cyder Farm, in Callestick, Britain, November 27, 2019. Dan Kitwood/Pool via REUTERS

LONDON (Reuters) - Prime Minister Boris Johnson looks on track for a large majority ahead of Britain’s Dec. 12 national election but tactical voting by opposition supporters could reduce or even eliminate it, according to an analysis of polls on Wednesday.

Johnson’s Conservative Party would win up to 366 seats out of 650 in the House of Commons, assuming supporters of opposition parties do not coordinate to counter the Conservative vote, according to the results from number-crunching firm Focaldata, published by anti-Brexit campaign Best for Britain.

However, Best for Britain said the outcome in many constituencies hung in the balance.

It identified 57 constituencies where as few as 4,000 pro-EU voters could cut Johnson’s seat-count to 309 if they voted tactically.

“Clearly this election is on a knife-edge, where even small changes in the number of votes each party wins could swing key seats,” said Focaldata founder Justin Ibbetts.

Focaldata’s numbers are based on 39,476 opinion poll responses, fed into a so-called MRP analytical model.

Another MRP model from analysis company Datapraxis on Saturday also showed Johnson on course for a hefty majority, but similarly subject to big risks from tactical voting.

YouGov is due to publish a closely watched MRP analysis for the current campaign at around 2200 GMT.