Lane Clark & Peacock, one of Britain's leading firms of consulting actuaries, says the company's 1994 accounts put the cost of providing Mr Brown's pension at pounds 555,000, but if the proposals had been in place it would have had to disclose a figure of about pounds 1.1m.
Similarly, Sir Richard Greenbury's own company, Marks & Spencer, would have to increase significantly the costs it discloses. The 1994 accounts show total pension contributions of pounds 500,000. Under the new proposals, these figures could increase to as much as pounds 2m.
The findings brought renewed Opposition outrage at the pay and perks of "fat cat" directors. Jack Cunningham, shadow trade and industry secretary, said: "These figures are yet another example of how boardroom remuneration is in many cases outstripping performance. Artificially boosting directors' pensions means that retiring directors receive more than they have earned the right to during their active careers.
"Labour is in favour of good reward for good performance, but abuses should not be tolerated."
The figures revealed by the proposed disclosures are much higher than those stated hitherto because they aim to show the value to the individual of the benefits promised rather than the contribution actually paid or accounted for under Accounting Standard SSAP24, as happens now. Such contributions can vary hugely, depending on the circumstances of the fund and the company. The true picture emerges from an examination of pension entitlements earned during the year, which is what the Greenbury Committee wishes to see disclosed.
However, the new disclosures would have no effect on the calculation of the overall cost to the company of providing pensions, which would still be calculated under SSAP24.
If accepted, the proposals could be implemented for companies reporting results at the end of this year.
Bob Scott, partner at Lane Clark & Peacock, said the approach would give investors a far better understanding of the issue, while reducing the scope for creative accounting in the way the costs are presented.
However, Gerry Acher, head of audit and accounting at accountants KPMG, warned that more work might need to be done before the guidance was made a listing requirement. He said there was a danger that the proposals might add confusion, "particularly where bonuses are pensionable and the resultant figures will sway around from year to year in a meaningless way".
Lane Clark & Peacock's findings follow the firm's publication of a survey last month, which found that 17 of the FT-SE 100 companies made negligible or no disclosure of directors' pension costs.
The other 83 annual reports showed pension costs averaging 11 per cent of remuneration, which the actuaries said falls "far short of the cost of providing even current service benefits, let alone the extra past service costs arising from large salary increases". The reality is that pension costs each year are likely to be over a third of the disclosed remuneration.
For a pounds 300,000-a-year executive director of an FT-SE company who has been promised a pension of two-thirds final salary on retirement at 60, the capital cost is around pounds 3.5m, says Lane Clark & Peacock.