The Government came under fresh pressure to guarantee the pensions of 450,000 current and former Royal Mail workers as trustees of its battered pension fund said that they were “very disappointed” by a ministerial U-turn on reform this week.

The trustees said they were seeking urgent talks with the company and the Government to ensure that more than £20 billion of pension promises to postal staff were honoured and secure.

However, a spokesman for the Prime Minister made plain that the plan for the Government to take over the pension scheme’s existing assets and liabilities would not go ahead without the accompanying plan to part-privatise the Royal Mail.

On Wednesday Lord Mandelson, the Business Secretary, announced in the Lords that both elements were being shelved because adverse financial conditions made it difficult to find a private sector partner prepared to buy a 30 per cent stake.

In doing so, he defused a rebellion by Labour backbenchers who were opposed to the part-privatisation, but reignited controversy about how a huge shortfall in the pension fund should be tackled.

The fund deficit is, by one measure, £5.9 billion, but is expected to have risen to about £9 billion when a triennial valuation is completed next year.

In letters sent to fund members yesterday, Jane Newell, the trustees’ chairman, said that she was seeking meetings to find a new way forward. She said that there was no risk to workers’ pensions in the short term.

However, in a letter to Lord Mandelson in February, she said that “the consequences could be very severe indeed for the Royal Mail Pension Plan and for Royal Mail itself” if the reforms were not put through. She said then that Royal Mail was “already balance- sheet insolvent” and that it was highly unlikely that the company could fund the shortfall from its own resources.

John Ralfe, an independent pensions expert, said: “There’s going to be a Mexican stand-off between the trustees and the company on the one hand and the Government on the other. But something’s got to give.”

Ms Newell has already raised the possibility of the Pension Protection Fund (PPF) being a safety net for members. However, that would require the extreme option of Royal Mail going into receivership. Besides, the PPF would be overwhelmed if tapped to rescue a fund as big as Royal Mail’s.

Poor investment returns and rising longevity have worsened the deficit. Royal Mail began to tackle the problem last year, closing the scheme to new recruits, watering down terms for accruing future benefits and lifting the retirement age. However, this does nothing to reduce the cost of retirement payments already accrued.

Royal Mail profits are likely to be swamped by the sums that it has to put in the pension fund.

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