Treasury won't tell us costs of Theresa May's Brexit

The publication by the Treasury of its forecasts of the economic impact of Theresa May's Brexit deal, versus no-deal and staying in the EU, has been keenly awaited.

But it turns out that what we will read, probably on Wednesday, will be almost irrelevant.

Because what the Treasury has modelled is not the deal actually struck on Sunday by Theresa May, but her Chequers plan.

And, as you will be keenly aware, the rest of the EU has rejected her Chequers combination of the UK staying in the single market for goods and the dual-tariff customs territory the Facilitated Customs Arrangement.

Prime Minister Theresa May making a statement in the House of Commons in London on Brexit on Monday. Credit: PA

In other words, we will be asked by the Treasury to compare two scenarios that the PM herself admits could yet happen - particularly no Brexit and a non-deal Brexit - with one scenario, Chequers, that cannot possibly happen.

In fairness to the Treasury, it was impossible for it to model the deal that was actually done at the weekend because in respect of the UK's future trading relationship with the EU, Theresa May's Brexit is myopic to the point of being semi-blind.

The Political Declaration on the outline of the Future Relationship with the EU, signed by May and the other EU 27 leaders, allows for a range of future trading option, from an almost conventional Free Trade Agreement close to Canada's with the EU to something that is nearly but not quite as ambitious as Chequers.

So if you want to actually know the economic cost of the deal, Theresa May will attempt to sell to MPs on December 12 start with what the Treasury projects as the economic cost of Chequers compared with staying in the EU and then knock off a bit more income.