Sir Keir Starmer's survival strategy has notched up yet again, with further defiant talk aimed at raising the political barrier as high as possible for his potential challengers to clear.

I have sat down with the prime minister twice in the past week. First, last Friday, when I was invited into 10 Downing Street for a longer interview than usual.

And again today, for a shorter question and answer session as part of a round of interviews with broadcasters at the G7 summit in the French Alps.

He had clearly decided that even if he wasn't asked directly, he would, as he put it to me, "gently point out that we do have a Manchester mayoralty by-election which will follow immediately, if Andy Burnham wins the by-election".

This was a reference to the fact that the mayor of Greater Manchester cannot be a sitting MP, meaning a Burnham victory would create a vacancy in one of the plum roles in devolved English politics.

Starmer was effectively saying that Burnham's first priority if he wins should be to ensure Labour holds the mayoralty, rather than plotting his own route to No 10.

In other words, the prime minister was seeking to play for time – and point out to the Labour Party that Burnham caused the Makerfield by-election, and will trigger another one if he wins.

Even if Labour hold both, they are only back to where they started in terms of the seats and offices they hold.

Team Burnham, for their part, dismissed this idea of delay as "absolutely ludicrous".

Cabinet role

They also added, bluntly, that Labour would be more likely to lose a future mayoral contest if there hadn't been an attempt to remove the prime minister by then.

And, after Starmer suggested he'd be open to giving Burnham a job in his cabinet, Burnham's team made it clear this was an offer they would reject.

This is hardly surprising – Burnham's whole pitch in the by-election has been about "changing Labour", and it would be a struggle to claim he could do this whilst accepting a role in the current government.

Meanwhile, former health secretary Wes Streeting is publicly pondering how soon he may trigger a contest.

And all this on the eve of the most consequential by-election in decades, whatever the outcome.

If Burnham wins, he'll instantly be seen by many as the favourite to become the next prime minister.

If he loses, Labour could spiral towards new depths of gloom – with an imminent leadership tussle still likely.