The vast majority of preliminary tax returns contain errors which favour the Treasury. Taxpayers are urged to check their returns very carefully.

Gestha, an association representing Tax Office staff nationwide, says that almost 70 per cent of the pre-completed forms sent out to households by local branches have mistakes in the calculations, particularly in the allowances to which the recipients are entitled to offset their tax payable.

The new system, whereby the Tax Office fill in the forms directly using information supplied by employers and banks etc, is a huge success because it just takes a phone call or an Internet visit to confirm the end result and the rebate (in most cases) is automatically sent within a few weeks.

However, Gestha says people should check the figures carefully because many of the calculations underestimate the size of allowances. One of the main errors, concerns mortgage relief for houses purchased before 2006.

The allegations have been strenuously denied by the authorities, who say Gestha’s claims are grossly exaggerated.