There is a lack of understanding amongst many, including the Treasury, regarding the impact that that changes to the rules governing SIPPs will have.

The article suggested that Gordon Brown may have to reconsider the range of investments available via a SIPP, including proposals to allow residential property to be held in the tax shelter of a personal pension from next April. Buying residential property through a SIPP is expected to reduce the effective cost of a buy-to-let property by 40%.

Ros Altmann, a consultant who advises the government on pensions, told the FT: "I just dont think they have a clue how much money is waiting to go in here . . . theres a hell of a lot."

Lord Oakeshott, a Liberal Democrat Treasury spokesman, is now set to write to the Chancellor warning him: "If you do not act to safeguard the tax base now, I fear current tax leakages will turn into a tidal wave next year."

While Lord McIntosh, a former minister, told the Lords in January: "Any additional cost to the exchequer will depend on uncertain behavioural responses to these reforms - whether or not individuals who will be able to invest in a wider range of assets choose to invest more than they do now."

Lord Oakeshott said: "Translated into English, this means we havent a clue."