The Chequers Brexit plan raises “very serious questions of constitutional propriety” because it was drawn up by officials in the Europe Unit in Downing Street who made “undertakings to Brussels” without proper political oversight, according to Stewart Jackson, who was Chief of Staff to the Brexit Secretary, David Davis.

Jackson describes how he and others in the Brexit Department attributed the delays in publishing the Brexit White Paper to incompetence: “We made the mistake of assuming that what was happening was cock-up when it was conspiracy.”

And he laments that the plan is far less good than it could have been: “It’s almost as if we’ve been polishing an Aston Martin and we turned up in a Trabant.”

The interview was conducted yesterday morning, and opens with Jackson dismissing as “barking mad” the call by Anna Soubry and others for a national government: “I’m afraid they speak with forked tongue. Because they say that they don’t want to stop it [Brexit], but they conspire at every juncture to impede it, to traduce it, to undermine it.”

ConHome: “Did you happen to catch Anna Soubry in full flight on the radio this morning, calling for a national government which can’t be held to ransom by the extremes? What do you think of Soubry’s role in all this?”

Jackson: “Well I almost feel like Frankenstein, because I actually mentored Anna Soubry.”

ConHome: “Oh!”

Jackson [MP for Peterborough from 2005 until losing his seat in 2017]: “I mentored three candidates in 2009 – Karl McCartney, Andrew Bridgen and Anna Soubry. She is clearly discombobulated by Brexit. This Fools’ Crusade, which Dominic Grieve has also been opining on, you know, we either have a break-up of the political system in the UK or we get Brexit, it’s barking mad.

“They represent no one really but themselves and maybe a dozen others. And I think it’s untenable for them to be saying that. Because when they were on the up, like they were going to win amendments and canvas the Government, they were absolutely fine with our parliamentary system.

“Now it looks as if the Government might be pushed towards a more realistic and authentic Brexit, now we’re seeing the end of civilisation. Politely put, it’s double standards.”

ConHome: “And impolitely put?”

Jackson: “Well, you know, she has said at various times that she respects the voters, that she respects the referendum, she voted for Article 50, she voted for the EU Referendum Bill in 2015. You don’t keep moving the goalposts because you’re emotionally unable to accept the mandate of the people.”

ConHome: “They claim they don’t want to stop Brexit.”

Jackson: “Well I don’t believe them. I’m afraid they speak with forked tongue. Because they say that they don’t want to stop it, but they conspire at every juncture to impede it, to traduce it, to undermine it.

“And I think they need to face reality that it is going to happen, it’s virtually impossible now for it legally to be prevented, and they should get on board and make the best of the decision even though they didn’t support it.”

ConHome: “And even though they believe it will be a catastrophe?”

Jackson: “Well they believed Osborne’s dystopian vision and none of that has come to fruition.The more they keep telling business that WTO is a disaster, no deal would be appalling, it’s not good for the body politic.

“The level of rancour and bitterness in her [Soubry’s] speech on Monday was quite frightening really. The idea that everyone who voted for Brexit is a double-breasted pinstripe suit Etonian trust-fund babe is nonsense.

“And even if Jacob Rees-Mogg has inherited wealth and is a rich man, that doesn’t really undermine the efficacy of his case. He’s a patriot, he believes in Brexit, he’s been reelected by the people of North East Somerset, he has as much right to articulate a view as she has.”

ConHome: “Now your former department, the Brexit Department, Steve Baker’s called it the Potemkin department, do you think that’s right?”

Jackson: “I think we made the mistake of assuming that what was happening was cock-up when it was conspiracy, and when we saw the definite strategy of preventing publication, pushing back on publication of the White Paper, from February, March, we assumed it was stasis of the Civil Service, it was incompetence, it was this obsession with write-round and getting everyone in the right place, and actually it wasn’t.

“We should have been aware – in February, they tried to bounce David Davis and the department to what I call Chequers 1.0, exactly what they wanted to do at that stage. And David Davis pushed back on that, said I don’t accept it, I don’t believe it’s the right approach, it’s not in keeping with the Florence speech, it’s not in keeping with the undertakings we’ve made in the general election and at Lancaster House, and it’s not acceptable – it won’t pass the Cabinet and it won’t pass Parliament.

“And whereas we thought that was the end of the matter, it was clear at that stage that there was a decision in the Europe Unit at Number Ten, at the highest level, to circumvent the department. And the imperative they had was to stop us holding the pen and producing the White Paper, because once we’d produced it, and it was really more or less ready to go in April, early May, that became our de facto UK policy and would be accepted by the EU.

“Now obviously there was a great degree of technical granular negotiation going on, the sort of non-sexy stuff, but the big-ticket picture, which was customs, goods, Northern Ireland protocol, non-regression clause, those things that were chunky and contentious, were constantly being held back, and it was clear at the end of course, before Chequers, that there was a reason for the bizarre decision not to publish the White Paper before the crucial June council, which was they absolutely had to stop us publishing it.

“And this gives rise, I think, to some very serious questions about the constitutional propriety of having an unelected unit, almost Nixonian unit in Downing Street, making policy, making without prejudice undertakings to Brussels, without real scrutiny and oversight by the Cabinet or the Strategy and Negotiations Sub-Committee. And for me that is a major issue which Members of Parliament should and must return to.”

ConHome: “So you think there has been impropriety.”

Jackson: “Well I think there’s been constitutional impropriety, in that to wilfully seek to circumvent the department and ministers is quite a serious issue. And it’s actually tragic, because we’ve wasted a huge amount of time on ending up in the cul-de-sac of Chequers.

“I think a lot of the Chequers strategy was predicated on losing the customs union vote, which of course we won by six. So everything has been held back.

“What the Number Ten unit was saying was one, we can’t win the customs union vote in the Commons, so we have to find a different way, customs partnership plus, and also that it’s non-negotiable. Suddenly out of the blue within a few weeks of Chequers civil servants, not politicians, say ‘Well, we’ve had a think about it, and basically that’s non-negotiable, so here’s one we prepared earlier for Chequers.’

“I just don’t think that’s a good way of running a government, because we could have been in a much stronger position of having legal texts for a free trade agreement, of making good arrangements for agency participation, we could have locked down a future security partnership on this, and now we’re in this terrible predicament, this real pickle politically, of what position Parliament and the Government are going to be in vis-a-vis the EU in October, which is pretty sobering.

“It was clear some Cabinet ministers were in on the operation and knew more about Chequers. We actually only found out about Chequers in its entirety on the Tuesday before the Friday, because it was only then we saw the details of what I rather inelegantly said to Robbie Gibb [Downing Street Director of Communications] was NCP [New Customs Partnership] pig with lipstick on, which he took great exception to, which we thought had been killed off because it didn’t have any friends.

“I said it was like the Rasputin of customs regimes – we shot it, poisoned it, had thrown it in the river, it still wasn’t dead. Robbie Gibb wagged his finger at me on that Tuesday and said ‘If I read about NCP pig with lipstick in the paper I know who it’s come from.'”

ConHome: “That rings a bell.”

Jackson: “Does it? Not from me. But having said that, I’m still quite fond of Robbie. I think he’s a decent guy and he’s got a tough job. I went to university with him, did the same course, known him 30-odd years.”

ConHome: “I can’t remember which university that was.”

Jackson: “Royal Holloway. And I’m very fond of Gavin [Barwell, Downing Street Chief of Staff]. Gavin was a great local MP. Let’s be honest, he’s steadied the ship in very difficult circumstances, and is doing as good a job as he can. But he can only work with the product you’ve got.

“It’s almost as if we’ve been polishing an Aston Martin and we turned up in a Trabant. We could have been really ahead of the game. We could have said here’s the free-trade agreement ready to go. Here’s the legal text ready to go. Here’s our relationship with the European Chemicals Agency. Here’s the legal text.

“We could have been so far ahead. Now we’re in this awful position as a supplicant going cap in hand to them, because the Chequers offer, even if it survives contact with the House of Commons in October, is sub-optimal.

“Personally I think on one issue alone for the Government it’s going to be very difficult, and that is the financial settlement. The idea that we can get MPs to sign a giant direct debit mandate without a definitive undertaking for a trade agreement – I just think it’s very unlikely.”