Pensions minister Steve Webb suggested that all parties will have to look at whether the elderly can receive the same support after the general election.
The triple lock ensures the state pension goes up in line with whichever is highest out of inflation, average earnings, or 2.5%.
Mr Webb stressed, in an interview with the Financial Times, that he believed the protection should be maintained.
"My view is it should be triple lock. To be absolutely clear, I would want to see that continue," he said.
"But we, as a party, will have to thrash that one out."
Labour announced earlier this month that it would strip wealthier pensioners of winter fuel payments if it returns to power.
Shadow chancellor Ed Balls also sparked controversy by signalling that pensions would be included in a cap on welfare spending - although he later insisted he still supported the triple lock.
Downing Street declined to give an explicit commitment to keeping the protection after 2015.