Boris Johnson’s determination to force a pre-Christmas election amounts to the biggest gamble of even his political life. Although the Tories have been ahead of Labour in every opinion poll bar one since he became prime minister in July, their lead has varied substantially.

Within the last 10 days, YouGov reported a 15-point Tory lead — enough for a comfortable 70-seat majority. ComRes, on the other hand, put the Tories ahead by just four points, which would condemn them again to be merely the largest party in a parliament they were unable to control.

An election may not give the Tories a clear majority LINDSEY PARNABY

While Jeremy Corbyn has worse ratings than any opposition leader since records began, Johnson’s own numbers are far from stellar. He has lower approval ratings after three months in office than