Making a vote in parliament one of confidence in the entire government used to be the nuclear threat in a prime minister’s armoury.

If their MPs rebelled they would have to accept the consequences not just of being disloyal, but of collapsing the government and precipitating a general election.

But unfortunately for Theresa May, her predecessor David Cameron removed that weapon from her. And so Downing Street’s threat to make a customs union vote in parliament an issue of confidence is little more than bluster.

The reason is the Fixed-term Parliaments Act that was brought in by the coalition government. This removes the right of the prime minister to unilaterally dissolve parliament and call an election. The only way that an election can be called