At some point this year MPs will vote on whether Trident should be renewed. It is politics, rather than policy, that will likely determine the timing. With Labour more divided on the nuclear question than any other, the Tories aim to inflict maximum damage on the opposition. Some want an early vote in order to wreak havoc ahead of the May elections, while others suggest waiting until autumn in the hope that the unilateralist Jeremy Corbyn may have changed party policy by then.

Urged at PMQs by Conservative defence select committee chair Julian Lewis to "do the statesmanlike thing" and hold the vote "as soon as possible", Cameron replied: "We should have the vote when we need to have the vote and that is exactly what we will do" - a reply that does little to settle the matter.

As I've reported before, frontbenchers have been privately assured by Corbyn that they and other Labour MPs will have a free vote on the issue. Just seven of the shadow cabinet's 31 members support unilateral disarmament, with Tom Watson, Andy Burnham, Hilary Benn and Angela Eagle among those committed to Trident renewal. But interviewed on the Today programme yesterday, after her gruelling PLP appearance, Emily Thornberry suggested that Labour may advise MPs to abstain. Noting that there was no legal requirement for the Commons to vote on the decision (and that MPs did so in 2007), she denounced the Tories for "playing games". But the possibility that Labour could ignore the vote was described to me by one shadow cabinet member as "madness". He warned that Labour would appear entirely unfit to govern if it abstained on a matter of national security.

But with Trident renewal a fait accompli, owing to the Conservatives' majority, the real battle is to determine Labour's stance at the next election. Sources on both sides are doubtful that Corbyn will have the support required to change policy at the party conference, with the trade unions, including the pro-Trident Unite and GMB, holding 50 per cent of the vote. And Trident supporters also speak of their success against the left in constituency delegate elections. One described the Corbyn-aligned Momentum as a "clickocracy" that ultimately failed to turn out when required.