The Association of County Councils has reacted angrily to the warning, which came in a letter last week.
It has been pushing for a standstill budget for next year and said a cut would put lives at risk.
'It is foolish for the Home Office to consider cutting civil defence grant. If a major flood or disaster strikes, the emergency services will find it hard to cope,' said Mick Warner, the ACC's fire and emergency planning chairman.
'County councils are being hit with a Home Office double whammy. It is planning to slash their funding before it knows how much money is available. And it's penalising them for local government reorganisation,' said Mr Warner.
The ACC said Hampshire CC would be particularly badly hit by the proposal to reduce county grants where unitary authorities have been set up.
The reorganisation of civil defence provision to cope with the split from counties of unitaries will also add to the strain.
'The government is risking public safety to save money. The Home Office must think again,' said Mr Warner.
He said officials were being premature in warning about cuts before spending plans, due to be announced after the November Budget, had been finalised.