When British Coal was privatized in 1994, an agreement was made for the government to cover pension payments, complete with inflation-related increases, if the scheme ever ended. In return, the state would receive half of any profit.

The plan was not posted to the members then and there sparked a bitter 30-year struggle to get the money back.

The government has since maintained the “Investment Reserve Fund”, a £1.5bn emergency pot built up from its share of profits, which would be the first port of call if the scheme fell short.

However, Miliband returned the entire fund to miners in a move he said would end an “injustice” and repay a “debt of gratitude”, with fellow minister Sarah Jones claiming Labor had ended a “scandal”.

This was confirmed in the Budget, along with compensation for those affected by the Post Office and infected blood scandals.

£1.5 billion is the same amount the government will save every year after remove winter fuel payments from 10 million pensioners, and this would fund around 39,000 nurses according to the charity The King’s Fund.

According to the publicly available documents, the scheme had a surplus of £1.5 billion in 2017 and a deficit of £225 million in 2020. Its latest newsletter also claimed a surplus of £1.1 billion in 2023.

With the investment reserve fund empty, taxpayers will have to fund any shortfalls in pensions. However, the government refuses to publish the 2023 valuation, which would reveal the scheme’s full financial position.

Shadow industry minister, Greg Smith, said: “It’s proof that Labor is financially incompetent. It’s all very well that Starmer says he wants to be an integrity government, (but) if they don’t even show the numbers when they hand out 1.5 billion pound, people are raising eyebrows and asking questions – and rightly so.

“Be transparent and act with integrity, or don’t and don’t be surprised if people doubt your intentions and motives.”

Conservative MP Bob Blackman said it was what he had come to expect from Labour, adding: “The fact is it’s a bit like this so-called ‘£22bn black hole’ that nobody can point to.

“So once again (Labour) are quoting figures without actually giving the details behind it and the public clearly have a right to know where the money is going and on what basis.

“They paint a broad picture and yet without any background or detail expect anyone to believe them. They may be right – if so, what do they have to hide?”

The Department of Energy Security and Net Zero dismissed the allegations, but said the rating would not be released “due to the sensitive nature of its content”.

A spokesman said: “We reject these figures and stand firmly behind the independent valuation of the Mineworkers’ Pension Scheme, which shows a surplus of over £1bn.

“We have agreed on the transfer of the investment reserve fund to scheme members, overturning a historic injustice and ensuring fair payouts for years to come.”

As well as returning the investment reserve, Labor has repeatedly pledged to review the other “unfair” profit-sharing arrangements for the scheme.

The party was accused of “treason” last month when it said there were no plans to return any money to more than 40,000 former miners in the British Coal Staff Superannuation Scheme, many of whom worked in senior roles alongside members of the Mineworkers’ Pension Scheme.

Successive governments have made a £4.7 billion profit from the Mineworkers’ Pension Scheme without ever having to pay in.