Royal Mail said its pension fund deficit has grown to £6.6bn and the costs of servicing the fund rose by £280m to £730m in its 2006-7 financial year.
Adam Crozier, chief executive of Royal Mail, said: "The £730m annual cost of servicing the pension fund clearly damages our competitiveness as we need to increase the price of our products and services to pay for it.
"Around 93% of our mail volumes comes from business customers and they should not have to pay for the increased cost of our pensions - and if we ask them to do so more of them will simply go to the competition."
The list of business customers to have defected from Royal Mail since deregulation in 2004 includes Sainsbury's, Npower and Lloyds TSB, and since the New Year, the BT and TV Licensing accounts.
In April Royal Mail will start a six-month consultation on its plans to close the final salary scheme to new members and also on ways to safeguard it for existing members.
Dave Ward, deputy general secretary of CWU, said: "The Union will use this time to marshal its opposition to the plans."
Today Royal Mail also revealed that the government, its sole shareholder, has signed off a financial restructure.
It agreed a 'phantom' share incentive scheme to run until March 2012, having ignored calls from Royal Mail's management for employees to receive real shares in the company.
The scheme was described as similar to that of the John Lewis Partnership.
All employees will receive an equal amount of free 'partnership units', currently worth 20% of the value of the business. When they sell their partnership units they will receive their share of the increase in the value of the business. Employees holding shares will also receive an annual profit share.
The maximum each employee can receive over the course of the five year period is £5,300.
The CWU's Ward said: "Phantom shares are nothing but jam tomorrow. They could result in phantom money and phantom jobs."
Also as part of the restructuring, the government will release £850m from Royal Mail's reserves into an escrow account to fund pension payments should the business fail. Royal Mail will also pay £150m into the account.
Royal Mail currently has undrawn borrowing facilities of £844m, which the government will increase to £900m on commercial terms, and supplement by another £300m on commercial terms.