At school, you can only tell the teacher so many times that the dog ate your homework. After a while, you have to produce it. Or admit you never did it in the first place. That is the position Boris Johnson finds himself in on Brexit. And whether it’s being accountable to parliament or to the public in an election after a vote of no confidence this week, it’s time he came clean on what his detailed plan is.

Theresa May’s mistake was to promise “Brexit means Brexit” but then negotiate a “vassal-state” deal, as Jacob Rees-Mogg described it, that was the worst of all worlds and more unpopular than the poll tax. Johnson quit his cabinet position as he felt so strongly that May’s Brexit deal was a bad one. Brexiteers said it was because her heart wasn’t in it as a remainer. So it’s hardly surprising there is little trust left in the Brexit process.

But now the leaders of the leave campaign, Johnson and Michael Gove, are running Britain and have the chance to deliver what they successfully sold to the electorate. So where’s the plan? Where’s the homework? Seemingly Dilyn, the new No 10 dog, has eaten it.

May’s cabinet published a white paper on her Brexit proposal, but from Johnson, Mr Brexit himself, we’ve had nothing. No sign at all of any plan, let alone a 98-page white paper.

Europe isn’t the enemy – demonising us is undermining Britain | Joris Luyendijk Read more

He told parliament last week that progress was being made on negotiations with the EU. But what exactly is he asking for?

The Brexit campaign never had a manifesto – it was largely limited to a bus advert. Johnson’s officials have made documented proposals to the EU already, yet the British people and their elected MPs are kept in the dark. We have a right to know what Johnson is negotiating on our behalf. Otherwise, people will reasonably conclude that there is no plan, or it is another worst-of-all-worlds Theresa May 2.0 plan and Johnson doesn’t want to admit it. No 10’s lack of transparency can only be a sign of weakness.

Play Video 1:09 Boris Johnson: we can leave EU on 31 October without breaking law – video

It should be especially concerning to Conservative activists in Manchester that no details of the government’s EU proposal will be available until after this week’s conference is over. What is so unpalatable that their own party leader needs to hide it from them?

I’ve served with Johnson in cabinet and worked with him for more than 10 years. It’s clear to me his strategy is dangerous and dysfunctional: get people angry with the judiciary, get people angry with parliament and then, after a fake negotiation, get people angry with the EU. Make the “great poisonous puffball of Brexit”, as Johnson referred to it last week, so toxic for Britain that people will desperately accept any version of Brexit, however damaging, to “just get it done”.

But it’s not the judiciary’s fault that Johnson unlawfully prorogued parliament. It’s not parliament’s fault that he hasn’t produced his plan, or details of how no deal would be dealt with. And it’ll be by design when his fake negotiation produces nothing or Johnson comes back from the EU with something remarkably like May’s original deal.

Boris Johnson is in charge. Failure will be at his door, no one else’s. Offensive and incendiary language intended to distract parliament and Britain won’t distract from the reality that he and his government have produced no detailed plan whatsoever.

A blizzard of spending commitments at this week’s conference won’t hide the fact that the Brexit homework’s late. It’s time for Johnson to hand it in to the British people, or admit he never did it in the first place. Surely Conservative activists in Manchester, alongside the rest of our country, deserve that, above all else.

• Justine Greening is the independent MP for Putney and a former Conservative cabinet minister