Director at The Institute for Fiscal Studies (IFS) Paul Johnson said just £35bn of cuts had already happened, with £55bn yet to come. He said that since the chancellor has successfully cut spending in the current parliament, it would be wrong to call the next tranche unachievable.
But he said there is a question of how they would be made.
"Is it a fundamental reimagining of the role of the state?" he asked at a briefing in central London.
He said that moving in the direction so far indicated, if health and pensions were protected, "the role and shape of the state will have changed beyond recognition."
London Councils expressed concerns about the further cuts ahead of the Autumn Statement.
Leader of its environment and transport committee Julian Bell, also leader of the London Borough of Ealing, told Horticulture Week he does not know how local authorities will "square the circle" of further budget cuts over the next five years - after already cutting "low-hanging fruit" - without seriously damaging services.
He said many councillors feared that it would be impossible to provide for more than statutory services social care and bin collection by 2020, unless something changes.