Changes to the tax disc system are making both buyers and sellers of cars pay Vehicle Excise Duty in the month of sale, motoring organisations claim.

The rules mean that existing tax cannot be transferred to new owners when a car is sold, so the remainder of that month's tax is essentially lost.

Equally, a buyer must pay for an entire month's tax, even if they buy a car part the way through a given month.

Some have estimated that the Government has made £38m in this so-called 'double-charging windfall'.

The new electronic vehicle excise duty system, introduced last October, does not work on a daily basis, but on a monthly basis.

Car owners caught without valid tax face large fines and may have their cars clamped or towed away.

Some motoring organisations suggest that the new system is unfair, in that up to double the tax may be being paid on a car in the month in which it is sold.

In February this year there were 8,741 clampings related to untaxed cars – up from 5,115 in February of 2014. That represents a 71 per cent annual increase, much of which is down to confusion over the new tax disc changes, it has been suggested.

There were more than 8,000 clampings in March of this year, too.

Someone buying a car must pay the tax for the entire month in which a car is bought. So even if someone bought a car on the 29th of April, they must buy tax for the entirety of April, and then monthly on an ongoing basis.

The changes could mean that buying a car will become especially common in the first few days of the month, in order to minimise this loss.