The Supreme Court has left Boris Johnson with zero credibility – and no way to deliver Brexit

The judges’ decision simply welds shut the metal box in which he is trapped

The impact of the Supreme Court ruling on Boris Johnson is about as damaging as it could be, given that Lady Hale could not sack him as prime minister on the spot. The court unanimously found that he misled the Queen, that his advice to suspend parliament was unlawful, and that parliament is therefore still in session.

But, curiously, the ruling has little effect on how Britain will or will not leave the EU. The prime minister was already in a worse position on Brexit than I think most people realised. This decision simply welds shut the metal box in which he is trapped. His only way out is to pass a deal through parliament by 19 October – the date specified in the law passed before prorogation to prevent a no-deal Brexit.

Now the desperation with which he will be trying to get any deal at all approved will be evident to all, making it all but impossible for him to succeed.

The ridiculous idea that Johnson could prorogue parliament again has been quashed emphatically by the court. Lady Hale said plainly that a normal prorogation before a Queen’s Speech would be four to six days. So that means parliament has to sit now until a few days before such an event, if it goes ahead.

Johnson was never as much in control of the timetable as he pretended. Now he has lost the only breathing space he had – the plan to keep MPs away from Westminster until 14 October.

He was already constrained by the act passed by Hilary Benn, which requires him to agree an extension to the Brexit deadline if he hasn’t secured approval for a deal at the 17-18 October EU summit.

The huge significance of the Supreme Court ruling is that it wipes out any prospect of Johnson’s being able to find a way round the Benn act.

It not only undermines the prime minister’s credibility but it makes it abundantly clear that the court will intervene quickly and brutally to prevent any attempt to subvert the will of parliament. And the one thing we have known since the start of this year is that there is a majority in parliament against leaving the EU without a withdrawal agreement.

But the court ruling makes it even harder to get a deal through parliament. Labour MPs sense that they have the prime minister on the run – Jeremy Corbyn made that clear with his impromptu response from the platform at the party conference in Brighton.

It will be harder now for Stephen Kinnock and his pro-deal colleagues among Labour MPs to vote for any deal that Johnson brings to parliament. They will be castigated by Remainers for bailing the prime minister out of his sinking premiership. That means we are likely to be heading for the biggest decision of Johnson’s short time as prime minister: on 19 October will he send the letter required by law asking the EU for an extension (which it is almost certain to grant), or will he resign? The choice is that stark.