The Royal Mail will have to double its pension fund contributions to £800m a year to keep its final salary pension scheme running, according to the company's chief finance officer Marisa Cassoni.
The company began to reduce its pension fund deficit last year with an annual payment of £140m after 13 years of making no contributions at all.
Cassoni said: "It would be 3p on the price of a stamp just to clear the deficit. It's the biggest financial problem we are facing."
The Royal Mail pension scheme is the fifth biggest in the country with 430,000 members - 250,000 of whom are already drawing their pension.
The news comes as the Royal Mail posted profits of £1 million a day, prompting unions to call for increased pensions contributions from the company.