Ms Harman said the pension was unacceptable to the public
Harriet Harman has said former Royal Bank of Scotland (RBS) chief Sir Fred Goodwin should not "count on" keeping his full £650,000 a year pension.
The deputy Labour leader described the pension settlement - agreed by the RBS board - as "money for nothing".
The sum was unacceptable in "the court of public opinion," she told the BBC, and the government "would step in".
All three political parties have attacked the pension payment after RBS made a record loss of £24bn.
But the Conservatives have accused Labour of "synthetic anger" about the pension row and said it had had a chance to block the deal.
The Liberal Democrats said ministers needed to be "careful" about suggestions they might seek to pass a law to retrospectively strip Sir Fred of his pension.
Sir Fred has refused to give up some of the money - worth a total of £16m - voluntarily despite pressure from Gordon Brown, saying the pension package was approved by ministers in October before he left the bank.
Ms Harman said Sir Fred should agree to waive some of the cash, saying this was the most "honourable" thing to do.
"Sir Fred Goodwin should not count on being £650,000 a year better off because it is not going to happen," she told BBC One's Andrew Marr show.
Ministers have said they would be prepared to take legal action and other unspecified measures to recover some of the money, saying the pay-out is inconsistent with the disastrous state that Sir Fred left the bank in.
Ms Harman declined to say how the government would achieve this but made it clear it would not tolerate the award as it stands.
"The prime minister has said that it is not acceptable and therefore it will not be accepted," she added.
"And it might be enforceable in a court of law, this contract, but it is not enforceable in the court of public opinion and that is where the government steps in."
Her comments followed a similar warning from Business Secretary Lord Mandelson who said Sir Fred was "personally responsible" for bringing RBS to the brink of collapse and did not deserve such rewards after such a "terrible misjudgement".
In a letter published on Thursday, Sir Fred argued that Treasury Minister Lord Myners had approved the pension payment after the banker had made other "financial gestures" such as waiving the right to severance pay and some share options.
But Lord Myners said he had not realised at the time that the pension agreement was not contractually binding on the RBS board and could later be altered.
Ministers also question why Sir Fred should be taking his full pension at the age of 50.
"I can't see how it is a pension because he is not retired," Ms Harman said. "It is money for nothing. It is a severance payment."
The Conservatives said the government had failed to deal with the matter properly when it had the chance.
"The government has been totally incompetent about this issue," shadow chancellor George Osborne told the BBC's Politics Show.
"They had the chance back in October to stop it and they incompetently failed to do it."
Sir Fred left the bank under a cloud in November
Mr Osborne said Lord Myners must explain why he "signed off" on the deal and why his version of events differs from that given by Sir Fred Goodwin or face questions about his future.
"I think his career is hanging by a thread."
Labour MP John McFall, chairman of the Treasury select committee, said Lord Myners had "questions to answer" about his role in negotiations over the pension.
An explanation of who authorised the deal was needed, Mr McFall said, adding that his committee would welcome a chance to question Lord Myners over the issue.
The Lib Dems said Sir Fred should only be entitled to an annual pension of £27,000, the same amount as employees of firms which collapse.
"Nobody disputes that Sir Fred should be deprived of his pension," said its Treasury spokesman Vince Cable.
"The only issue is what is practical."
The government pumped billions into RBS in October to keep it afloat and currently owns a stake of more than 70% in the bank.
RBS has warned that it will have to cut thousands of jobs in the next few years as it restructures its business.
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