Brexit creates a dividend or fiscal fund that can satisfy this policy need while still bringing the public finances to a safe debt/GDP ratio in the 2020s

The Treasury wants to keep the lid on public spending and limit tax giveaways until UK public finances are sound. This is a good aim, since the Government's debts are about 80pc of our national income when you strip out the effects of the Bank of England's operations in the money markets and the debt of RBS.

But in March 2019 we formally leave the EU; and probably by mid 2020 we are out of the transition period. We must have the right policies to make sure the economy thrives. For that the Treasury needs to have a positive mindset. Unfortunately, it seems quite determined to take the gloomiest view possible.

The Treasury has not retracted the assessments of Brexit it made before the referendum: long-term, that GDP would be reduced by any Brexit, most of all (7pc) by a clean Brexit with no EU trade deal, and short-term, that there would be a sharp and immediate recession. The short-term one has already been contradicted by events: the UK is still growing well. The long-term assessment has come under widespread attack; its protectionist assumptions are contrary to the Government's trade policies, and its empirical work was largely based on non-UK experience. Neither the Treasury nor the Chancellor have replied to these crucial criticisms.

Against this depressing background the Economists for Free Trade group has felt it necessary to do its own assessment of the budget outlook up to and after Brexit. There are four main reasons why we take a much more upbeat view of the future for the economy and the public finances.

First, there are the current economic facts: the economy is doing well. Growth according to the ONS is running at just below 2pc per annum. We think that when all the data is in the final revised GDP growth rates will be seen to have been running at 2pc-plus since the referendum.

That would fit with the data from the purchasing manager indices, CBI surveys and robustly growing employment. It also fits with the steady improvement in the public finances. You would hardly think from all the reported hand-wringing that public borrowing is running below the OBR's forecast for this financial year. The money coming in from revenues is growing more strongly, at 4pc, than the money going out in spending (at 3pc). It looks as if borrowing this year will be around 2pc of GDP.

Second, that steady trend in revenue growth looks set to continue up to Brexit as growth in money GDP stays around the 4pc mark, so that by 2020, borrowing will have got close to zero. When borrowing is this low, debt falls as a percentage of growing GDP. By 2021, regardless of Brexit, debt would have reached 75pc of GDP, well on its way towards the safe target level of 60pc, where the risks of a sharp interest rate rise causing a crisis become manageable.

Third, the official gloom about UK productivity growth stalling is misleading. There is a huge measurement problem for the quality, and so the productivity, of services. It is easy to measure the number of plates rolling off a production line; but how to measure the service you get from your mobile phone or your computerised booking agency? For example, according to ONS figures, productivity in education, healthcare and social services has been falling since 1998 at 4pc a year; this is an absurdity.

Unfortunately until we can get some grip on the measurement issue the only reliable figures for the economy's growth are money GDP  the money value of what we spend and produce.

Fourth and finally, there is Brexit and the massive boost that its economic revolution will provide by bringing free trade and world competition, home-grown pragmatic regulation and an end to subsidised, unskilled EU immigration. In the long term this raises our GDP by 7pc, brings down consumer prices by 8pc and boosts the living standards of the lowest income groups by 15pc.

From 2020, it takes gradual effect because of the usual time lags; by 2025 about 2pc of the full 7pc will have come through, with the rest due by 2030. The total effect on government revenues will be a rise of about 10pc, worth £80bn in today's economy but of course much more by 2030.

When you feed Brexit into the public finances, two things happen. First, the future path of debt is much lower relative to GDP, as the finances improve sharply with no change in assumed public spending; this permits the UK to get the debt/GDP ratio to 60pc faster and indeed below 50pc by 2027.

Second, there is a policy need to support the changing Brexit economy, to make the most of our new opportunities. Faced with chances to grow faster and penetrate new markets all around the world, UK competitiveness should be reinforced: tax rates should fall, infrastructure be improved, and public services made fit for a stronger economy.

If you put these things together, Brexit creates a dividend or fiscal fund that can satisfy this policy need while still bringing the public finances to a safe debt/GDP ratio in the 2020s. We show that we can afford to spend another £25bn a year from 2020, topped up by a further £40bn a year in 2025. This will be enough to spend another £33bn a year on public services and infrastructure by 2025, a lift of about 4pc in public spending; and also to cut corporation tax to 14pc, the top rate to 36pc with no additional rate above it, and the standard income tax rate to 18pc. This is truly a future to be optimistic about.