Darling told a Confederation of British Industry (CBI) conference last month that he would announce concessions to the 18% flat rate proposal "within the next three weeks".

However, it now appears that businesses will have to wait until the new year to find out what changes are planned for April.

They are likely to include the £100,000 tax relief on retirement sales already detailed, but commentators said that this delay must result in broader changes.

CBI director general Richard Lambert said: "He needs to get on with this decision urgently, as he promised at the CBI's conference a fortnight ago.

"People need to be able to make decisions about their businesses – whether to invest, or whether to sell up. This uncertainty mustn't be allowed to continue."

Mr Darling described the changes as "quite complex" and said that he wanted further dialogue with interested parties "before I finalise my proposals".