At first sight the move by Liberal Democrats and the SNP to break with Labour and call for an early election seems inexplicable.

With the Tories ten points ahead in the polls, an election could be the prime minister’s best chance to deliver his own form of Brexit with a more compliant parliament.

So why are the two opposition parties seemingly doing Boris Johnson’s bidding by offering him a path out of the current gridlock?

Behind the move is a mix of principled, self-serving and expedient calculations. The first calculation is the reasonable concern that without an election the EU is unlikely to grant the UK another long extension in January.

This time round France held out in a minority of one against the January