Royal Mail is under pressure to embark on a fresh round of cost-cutting to shore up its finances, amid signs that senior ministers are reluctant to use taxpayers' money to plug a massive pensions deficit of more than £4bn.

The issue of the state-owned postal operator's pension liabilities, the full scale of which have been exposed by accounting changes, has complicated an independent review of its future ahead of full market liberalisation in January. It means a decision on a controversial share ownership plan for employees, which has been fiercely opposed by union leaders and many Labour MPs, could be delayed.