The Treasury will have to reveal the details of the forecasts behind Gordon Brown's move in 1997 to tax pensions, and explain how the government expected it to affect retirement funds.

The Daily Telegraph reports that following a freedom of inforamtion request, the Information Commissioner asked the government why it has withdrawn the tax credit on dividends. The request centres on how this move impacted the dividends collected by pension funds.

Until now the Treasury has refused to release the information on the grounds that it related to the development of government policy and would undermine the way policy decisions were taken.

Shadow chancellor George Osborne said: 'Now Gordon Brown will be forced to tell us the truth behind his £5bn pension tax raid, and whether he knew the damage he was going to cause to pensions.'