‘Only the higher-than-expected numbers of migrants coming to the UK under the post-Brexit migration regime adds materially to prospects for potential output growth over the coming five years relative to the assumptions that we made in March.’

That’s from the Office for Budget Responsibility’s (OBR) assessment accompanying the Autumn Statement. It’s a pretty striking line: the state’s official analyst of the public finances says that the only good thing to happen to the UK economy since March is higher immigration.

How much higher? In March, the OBR produced an economic forecast that assumed net migration would run at around 130,000 a year for the next five years. Now it puts that figure at just over 200,000.

Why? The OBR says it now realises that Britain’s post-Brexit immigration system is fairly liberal, especially to non-EU migrants. Implied in its forecast is the OBR’s belief that ministers will keep it that way too.

Without net migration, Britain would currently be facing even bigger tax rises and even bigger spending cuts than it is now

The OBR gets privileged access to government thinking and planning; its assessment of future migration numbers is based, in part, on its conversations with the Home Office’s Migration Advisory Committee. So those numbers may be ‘just a forecast’, but they’re a forecast firmly grounded in the reality of ministerial decisions and intentions.

Here, I can only say something rather childish: I told you so. No doubt this immigration outlook will make some people unhappy. Justifiably, too. It isn’t what they were promised, at least by the current Home Secretary. Suella Braverman said she wants net migration to fall below 100,000 – though to be fair, she said that as part of the Truss government in October.

Nonetheless, I think a bit of outrage about all this is understandable, and I say that as someone who is pretty positive about liberal immigration policies. I don’t think the politicians who have adopted the migration policies favoured by people like me have done enough to explain and sell those policies to others who don’t take my view.

I am also aware that some unhappy observers of immigration note the difference between GDP and GDP per capita. Their argument is that adding foreign workers to the British economy may make the cake bigger but it also cuts it into more slices, meaning there’s no tangible benefit to people in the UK.

It’s a respectable argument but it overlooks something big and important: the public finances. When people work a job, they pay taxes on their income and on the money they spend. Their employer pays tax on those wages, and on any profits they make selling the stuff the worker makes or provides. When a job isn’t done, those taxes don’t get paid. So, more work means more economic activity and more tax.

Again, people have strong feelings about tax. But the Autumn Statement is a simple reminder that the British state spends more money than it raises in taxes. To close that gap at least partially, politicians will reduce some bits of spending, and increase some taxes.

Overall, however, OBR assessment makes a very important point about immigration. More migrant workers mean more economic activity which means more tax revenue than would otherwise have been received. What this all translates to is that those cuts and tax rises announced yesterday are not as big as they would have been without those migrant workers. The OBR reckons higher immigration raises between £5 billion and £6 billion a year in taxes, money that the Treasury doesn’t have to find elsewhere. That’s about 1p on the basic rate of income tax.

So, the critics are right: there is more to the immigration story than just GDP. There are also the public finances, which are in a better condition than they would have been because the Government is taking a relatively liberal approach to migration.

To quote the OBR again: ‘The rate of growth in potential output in the final year of the forecast is unchanged since March, albeit with a larger contribution from net migration offsetting slower growth in productivity.’ In other words, things would have been even worse without that increase in migration.

So what are the politics of this? Largely, it means pain for the Conservatives. The migration issue makes clearer than ever that post-Brexit Conservatism is split between economic liberals and social conservatives. That division is longstanding, of course, and hasn’t yet torn the Conservative party in two. But I suspect the divide will become a lot more visible as this parliament continues.

As for the next parliament, who knows? It doesn’t seem fanciful to wonder if a Conservative party that suffers a major defeat at the next election can actually continue to be large and flexible enough to contain people who think that Britain must be open to foreign labour for economic reasons, and people who think it should not be. The return to prominence of an anti-immigration insurgent party seems perfectly plausible over the next few years. While, generally speaking, the British public is growing steadily more relaxed about immigration, there may be a constituency among Conservative-inclined voters for a much harder line on immigration.

Can those voters ever be reconciled to a liberal stance on immigration? I don’t believe that lecturing people worried about immigration on the economic benefits it can bring is the answer here – we’ve seen what happens when clever Westminster people condescend to the public over emotive topics such as this. But nor do I think that the liberal side of this debate can simply give up on its economic arguments, and those arguments became stronger this week.

Without net migration, Britain would currently be facing even bigger tax rises and even bigger spending cuts than it is now. This is a good news story, and it should be told more often and more loudly – especially by the Conservative ministers who have based their economic plans on the benefits of immigration.

They might start by asking the public a simple, frank question: would you rather have immigration running at its current levels, or slash it and pay an extra penny on income tax?

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