I don’t know if Boris Johnson is as surprised to be prime minister as those who’ve known him for 20 years, and worked for him when he was Spectator editor, and became incredibly grumpy at his seeming pathological inability to make up his mind.

But it all felt a bit unreal and disconcerting to see Johnson at the dispatch box today in the Commons – even though I’ve been calling him a shoo-in for the job for months. I presume the sense of detachment will dissipate.

One thing Johnson has made up his mind about, seemingly, is that – in the words he uttered to MPs – any negotiated deal to leave the EU ‘goes by way of the abolition of the backstop’.

He said the same thing during the leadership campaign.

But it matters that he confirmed in his very first speech as PM in parliament that he wants the Withdrawal Agreement that Theresa May negotiated with EU leaders reopened and the lengthy section ripped up that is designed to keep open the border on the island of Ireland.

The EU has responded promptly, or rather the European Commission president Jean-Claude Juncker has done so.

In a phone call this afternoon, Juncker told Johnson ‘the Withdrawal Agreement is the best and only agreement possible – in line with the European Council guidelines’.

In other words Juncker slapped Johnson down, on just his second day in office.

Johnson and his team will try to go round the back of Juncker – and his negotiator Michel Barnier, who was just as dismissive of what Johnson wants in an email to EU diplomats – by negotiating directly with EU government heads.

In other words, Johnson hopes the European Council will change its guidelines to allow the painfully and painstakingly Withdrawal Agreement to be ripped up.

That could happen.

But my own Brussels sources say that the European Council will not be moved. ‘That will simply not happen’ one told me.

’27 [government heads] will not want to go there. We seem to be back in the era when the UK thought Brexit would not damage UK influence on EU affairs.’

So at the risk of being one of those ‘doubters, doomsters, gloomsters’  whom Johnson excoriated on Wednesday from the steps of Downing Street, the more important part of the PM’s statement today was probably his disclosure that he has instructed his de facto deputy Michael Gove to make preparations for a no-deal Brexit ‘his top priority’ and the cabinet secretary Sir Mark Sedwill to ‘mobilise the civil service to deliver this outcome.’

We are witnessing the irresistible force of Johnson coming up against the immovable object of Brussels and the only possible consequence, if Johnson is to honour his pledge to remove the UK from the EU by October 31, is departure without a negotiated settlement.

Which, as I’ve been banging on about for weeks, means we are heading for an autumn general election, since parliament can and will frustrate a no-deal Brexit unless there is a popular mandate for it.

Robert Peston is ITV’s Political Editor. This article originally appeared on his ITV news blog.