North Yorkshire Police may have to foot a £3.8m bill to cover the cost of staffing its operation at Drax Power Station last summer.
Seven forces were drafted in to help police the operation
Seven forces were drafted in to stop environmental protesters shutting down Britain's biggest coal fired power station in August.
The police authority said it would normally have expected to contribute £1.3m - 1% of its revenue budget.
But the Home Office may be unable to cover the remaining cost, it said.
North Yorkshire Police Authority treasurer, John Kirman, said the force could have to pay the full cost of the operation.
He said: "The Home Office obviously has insufficient funds to go round.
"We have had a verbal indication that they are finding it extremely difficult to make a contribution towards the remaining £2.5m but that may change.
"It is not going to make any difference to the way we are today, it just means we will be challenged seriously in future years.
"We have reserves we can use in the short term but it leaves a very big hole in the long term which we will have to fund either by efficiencies or by increasing precepts in future years."
The authority was due to meet on Monday to discuss the issue.