The deficit in the Royal Mail's pension scheme could have "devastating consequences" for the business, according to a letter seen by the BBC.
The Royal Mail's chair of trustees, Jane Newell, has warned the pension scheme deficit is likely to rise sharply from its current £5.9 billion, unless a minority stake in the business is sold off, with "devastating consequences", the BBC reported.
Without a partial sale, the pension scheme may have to be wound up and members' benefits would be halved.
Although the government's Pension Protection Scheme would protect up to 90 per cent of members' benefits, Ms Newell said the resulting increase in levies could put an "intolerable strain" on other UK pension schemes.
Ms Newell said the recommendations in the Hooper report, which proposed solutions for the service including part-privatisation, should be implemented.
If the government is successful in its proposal to sell off 30 per cent of Royal Mail to the private sector, it will take over responsibility for the pension scheme.
However, unions and many MPs have reacted with anger to the plan to privatise part of the business and the bill will be met by fierce opposition.
Nearly 140 Labour MPs oppose the motion, and the chief whip has reportedly warned Gordon Brown he will have to rely on opposition MPs to support the bill, which will be presented on Thursday.
Unions are also opposed, arguing the business can remain as it is, and are staging protests today.