George Osborne has reached agreement with seven Whitehall departments on savings he wants made in 2015.
The chancellor said he had found 20% of the £11.5bn he wants to cut spending by in the year from April 2015.
Justice, energy and communities are among the departments agreeing to "significant savings", he said, adding that health, schools and foreign aid would be protected from cuts.
Labour says the government is sticking to policies "that are badly failing".
Mr Osborne told the BBC that no chancellor had got so far in agreeing so many plans so far ahead of a spending review, which is due towards the end of next month.
The spending period in question covers the month before the expected date of the next general election, scheduled for May 2015, and the year after.
The departments that have agreed cuts of between 8% and 10% are: the Ministry of Justice, the Department for Communities and Local Government, the Department for Energy and Climate Change, HM Treasury, the Cabinet Office, the Foreign and Commonwealth Office, and the Northern Ireland Office.
Mr Osborne said the scale of the savings expected was "difficult" but necessary to reduce government borrowing and to ensure money could be found to spend on the "nation's priorities", such as the health service.
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DEPARTMENT CUTS AGREED
Ministry of Justice
Department for Communities and Local Government
Department for Energy and Climate Change
Foreign and Commonwealth Office
Northern Ireland Office
Analysis: Osborne's spending challenge
"The fact we have got big departments like the Ministry of Justice signed up to 10% reductions shows we are on track and there is a cabinet will behind delivering these necessary savings," Mr Osborne told BBC Radio 4's Today programme.
Negotiations over savings are continuing with other big departments, such as the Ministry of Defence and the Home Office.