Pension blunders to be explained
 Affected pensioners will receive a letter outlining any changes to payments |
Tens of thousands of retired NHS, armed services and other public sector staff will learn later how their pensions came to be overpaid for decades.
The Cabinet Office will release a statement on the overpayments thought to total tens of millions of pounds.
Letters are being sent to pensioners explaining how they will be affected.
The government has said the money need not be repaid but thousands still face pension cuts from April. The Tories urged ministers to "get a grip".
'Excessive'
The statement had been due at about 0930 BST, but it may now not be published until the afternoon. It is not clear why it has been delayed.
It is understood about 5% of retired public sector workers, from all over the UK, are involved in overpayments.
Asked about the situation at his monthly press conference, Conservative leader David Cameron said: "You have to ask why this was allowed to go on for such a long time.
"This government has been in power for a lot of years. They have to get a better grip."
The issue was raised in the House of Commons by Liberal Democrat Treasury spokesman Vince Cable, who was informed about the problem 10 days ago by a BBC journalist.
He told MPs he had held off saying anything until Monday at the request of Sir Gus O'Donnell, the head of the civil service, who had asked for time to alert pensioners involved.
Mr Cable said the journalist had understood that the company Xafinity Paymaster - a former public body which pays out public sector pensions - had been "paying out excessive public pensions to hundreds of thousands of public sector pensioners".
He added that former armed services personnel and NHS staff were likely to have been affected.
Mr Cable told BBC Radio 4's Today programme: "We need to establish the full scale of the problem...
"The issue is how much money is involved. I think we are talking about tens of millions of pounds, maybe 100 million."
Mr Cable warned the government not to change its mind and ask pensioners to return money, adding that it had been "brutal" in the past when dealing with tax credits.
He said: "This is small beer by comparison with the banking bailout but it's a lot of money to a lot of people."
Why should retired public sector workers pay for yet more government mistakes? Pensions should not be cut
One BBC website user claims he received a letter on 5 December demanding repayment within nine days of nearly £16,000 for overpayments on his armed forces pension over the past 12 years.
Others users have contacted the BBC voicing concerns that already meagre pensions would now be reduced further.
On Monday, Mr Darling said the money would not have to be repaid, but he added: "It will be necessary to adjust what's paid for the future. It does need to be put right from next year."
Scottish National Party work and pensions spokesman John Mason said it looked like "an extremely serious issue" and described Mr Darling's comments as "very worrying".
He said: "Pensioners must not lose out. We need to know exactly what has happened and exactly what the UK government plan to do."
