Are the Downing Street dominoes about to fall?

At lunchtime on Thursday, my phone pinged. It was a video message from a Labour source, a gif of a shadowy hand flicking over one domino, which knocked over another, then another, then another. Then hundreds, then thousands, came tumbling down.

It was obvious what my contact was suggesting, half an hour or so after the shock resignation of John Healey. Could the exit of the now former defence secretary set off a chain of events that would lead Sir Keir Starmer's operation to fall over too?

The resignation was a disaster for Downing Street for many reasons. One cabinet minister told me everyone would be "shaken" by Healey's exit. Another insider joked grimly: "It's been a really hard week – stronger words could be used."

For the defence secretary to say publicly that the prime minister's decisions were putting the country at risk is about as bad as it gets. The top responsibility of any government is to protect us. For a senior government figure to say the prime minister's choices were making us less safe (and he does put it that strongly), is serious damage.

Second, Healey is about as loyal a Labour politician as you'd ever care to meet. For him, in particular, to quit really is a brutal judgement of the government. You can be sure he'd have tried everything in his power to make it work. Third, as one former Labour cabinet minister suggests, it illustrates that "Keir has never got control of the Treasury, even though he's meant to be in charge".

Healey's letter said , carefully, that No 11 had been "unwilling" to find the money for defence, but No 10 had been "unable" to make it happen – a real dig at his lack of authority.

And it's all taking place in an unforgiving context. Labour has already put the prime minister on notice, with dozens of MPs saying he should quit and at least two leadership contenders raring to go. But his record on security and managing foreign allies is often cited by the remaining Starmer loyalists as the reason he has to stay.

"What does Andy know about defence?" or "Can you imagine Wes handling Trump?" These are the kinds of retorts I hear when there are questions on whether the prime minister can really stay. But Healey's departure has just blown a giant hole in that remaining flank of protection.