Britain could gain a £65 billion windfall from Brexit if the Chancellor seizes its "economic opportunities", according to a report to be published this week.

A paper produced by a group of Eurosceptic economists warns that official forecasts are failing to question "negative" analysis of the effects of Britain's departure from the European Union published during last year's referendum campaign.

Instead, the 16-strong Economists for Free Trade group said its own modelling, based on Brexit taking place in 2020, showed that growth would improve, wages would rise, and unemployment fall.

A resulting "Brexit dividend" could be used to ease spending on public services and improve the competitiveness of the economy through tax cuts, it added.

The group, said its 'Budget for Brexit' report analysis factored in "long-run gains" including a fall in prices after abandoning EU tariffs on goods from outside of Europe, "improved export performance", less red tape, and "an end to the annual EU subscription of £10 billion".

"The effect of all this is to push growth up to nearly 3 per cent per annum by the mid 2020s," the group said. The report, written by Patrick Minford, a professor of applied economics at Cardiff University, predicts that state borrowing and the accumulated national debt will fall to create a budget surplus by 2021.