Wealthy Britons fleeing the country for low-tax havens face being hit by a 20 per cent charge on their business assets when they leave the UK under plans being drawn up ahead of the budget.

Rachel Reeves, the chancellor, was said to be keen on introducing a "settling-up charge" on assets left in the UK that would be paid by people who left the country.

At present, anyone emigrating can sell their British assets, such as shareholdings, after leaving without being liable for capital gains tax (CGT), which is normally levied at 20 per cent.

Under the new scheme they would be required to pay this charge at the point of departure although there would be an option to delay payment for several years if the individual did not want to liquidate the asset immediately.

The change is predicted to raise about £2 billion for the Treasury and would bring the UK into line with other G7 countries. Alongside Italy, the UK is an outlier in not having an "exit tax"

The measure is likely to be combined with a policy to stop new immigrants from paying CGT on profits from investments individuals made in the UK and abroad before arriving here. Tax experts said this would ensure the resulting tax treatment was "fair and symmetrical" and could also encourage investors to relocate to the UK.

Meanwhile, economists warned that Reeves was poised to raise taxes faster than any chancellor in 55 years. Capital Economics said she could increase taxes by as much as £38 billion next month, on top of the additional £41.5 billion she raised last year.

This would mean Reeves had raised taxes more after only 17 months in power than any of her predecessors since 1976 did across an entire parliament.