The Institute for Fiscal Studies said finance minister Philip Hammond's budget on Wednesday fell into a familiar pattern of looser policy in the short-term while promising deeper cuts several years ahead.
"Taking the figures at face value would probably be a mistake, though. As the realities of 2022-23 hove into view, the purse strings are likely to be loosened once again," IFS director Paul Johnson said.
"The chances of getting to budget balance by the mid-2020s look remote," he added.
The IFS said that based on current rates of debt reduction and sluggish official economic growth forecasts, Britain would not cut debt as a share of gross domestic product back down to its level before the 2007-08 financial crisis until the 2060s.
"This assumes no recessions for the next half century," Johnson added.