Campaigners and MPs say the chancellor did not go far enough in his commitments to former mineworkers in his Budget.
Mineworkers say they lost out after the Conservative administration took over the miners’ pension scheme when British Coal was privatised in 1994.
At that point it was guaranteed by ministers that the value of the fund would not decrease – but the government was allowed to keep half of any investment surplus.
On Wednesday, Chancellor John Healey said the 50-50 agreement would end and surplus cash would be paid to pension holders, but stopped short of announcing the release of remaining cash held in the pension scheme.
Prime Minister Andy Burnham said the commitment to former miners was “in no way diminished”.
Former chancellor Rachel Reeves agreed in the last two Budgets to release part of the fund and pay-outs from the two pension funds – the British Coal Staff Superannuation Scheme (BCSSS) and the Mineworkers’ Pension Scheme (MPS) -have since been made.
However, campaigners have been demanding the government release the remaining £1.2bn held in the pension fund.
In a letter to the chancellor, the Coalfield Group of Labour MPs said that money represented “the ill-gotten gains of a broken, unjust system” and urged the government to approve its release.
BBC News has been told there were tense conversations between Labour MPs and Treasury ministers in the day before the Budget.
The letter added: “We understand the fiscal pressures on the Treasury, however in light of the facts, we cannot see a justification for withholding money that belongs to the scheme members alone.”
In his Budget speech, Healey said some work had been done in relation to the miners’ pension fund, but he added the job was not finished.