Boris Johnson has just announced that he wants an election on 12 December. But he’s going to need cooperation from MPs outside his own party to do it.

It’s not actually in the Prime Minister’s gift to call a general election.

That power was stripped from Number 10 by the Fixed Term Parliaments Act (FTPA). The law was introduced in 2010 as an insurance policy for the Liberal Democrats, who worried that their Conservative coalition partners would call a snap election once the opinion polls were in their favour.

The FTPA says two thirds of MPs must vote in favour of an election before one can take place.

With a working majority of minus 45, that’s going to be tricky for Mr Johnson. He needs 429 MPs to back him, but there are only 288 members left on the Conservative benches. So he’ll need a decent chunk of MPs from other parties to support the motion.

Alternatively, the government could table a one-line bill that says something like “notwithstanding the Fixed Term Parliaments Act, there will be a General Election on 12 December 2019.”

The advantage of this for Mr Johnson is that he’d only need to get a simple majority of MPs to support it – i.e., 50 per cent plus one.

But there’s a catch: a bill of this kind could be amended by opposition parties. MPs have already mooted the idea of an amendment to allow 16 and 17-year-olds to vote.

Jeremy Corbyn has said he’ll help trigger an election once the threat of a no-deal Brexit on the 31 October has been eliminated. It’s possible that Labour or other parties would table amendments to make sure this is the case.