Some pensioners with spare rooms will be hit by reductions in housing benefit under what critics dub the Government's "bedroom tax", the Department of Work and Pensions confirmed.
Existing claimants in homes where someone of working age also lives have been spared the welfare cut, but new claimants under the Universal Credit scheme will face deductions.
But Work and Pensions Secretary Iain Duncan Smith insisted that no pensioners would "suffer" and that the future application of the rules was yet to be finally decided.
Shadow work and pensions secretary Liam Byrne told the Sunday People, which uncovered the different status of new claimants, that the Prime Minister needed to "get a grip".
"Ministers promised to protect pensioners from the bedroom tax, but thousands are set to lose out.
"David Cameron had better get a grip fast, before this scheme descends into total chaos."
A DWP spokesman said: "We want to reassure pensioners claiming housing benefit that they will not be affected by this policy now, or when Universal Credit begins, even if one partner is below pension age."
He confirmed however that this would not apply to new claimants after that time.
A poll for the newspaper found that voters believed - by a majority of 46% to 35% - that the principle of the change was "only fair" at a time of economic austerity.
But clear overall majorities said that its implementation should be delayed for a rethink on how to implement it (52%), that it should only apply if people refused smaller homes (60) and that it should not apply at all to Army families with children away on active service (77%).
Duncan Smith denied the government had finalised its plans: "The reality is that they are looking at Universal Credit further down the road and they have asked a specific question about that and we have yet to set finally what we do with that group," he told BBC1's Andrew Marr Show.
"It is not our intention that pensioners in any way will suffer under this particular thing.
"It is not about punishing people; this is about trying to reallocate the housing. It is about getting the balance right."