Sellafield's £9m tax rebate hits Copeland Council
A council must use £9m of reserves to cover a business rates overpayment by one of the biggest firms in its area.
The Nuclear Decommissioning Authority (NDA), which runs the Sellafield reprocessing site, has won its appeal against the original valuation.
Copeland Council had said it was "unfair" it had to cover a bill paid before rates were decentralised by the government in 2013.
The government has been approached for comment.
Copeland elected mayor Mike Starkie said, had the NDA won its appeal before business rates were localised, the government would have had to pay the rebate.
Council 'guinea pig'
"It's a complex financial situation and, really, we're the only council in the UK, with the exception of the council that's got Heathrow airport, that could be impacted in this way," he said.
He said the council had been a "guinea pig" for the rest of the country and would lobby the government to help councils in the same situation.
Copeland Council, the government and Cumbria County Council must return £31.5m.
More than 70% of its business rates come from Sellafield Limited, a council spokesman said.
The NDA and Sellafield said they would work closely with the council "in order to understand the implications for all parties".
The council had kept money in reserve in case the appeal was successful, Mr Starkie said.
There would be no direct impact on jobs and statutory services but some other projects may have to be put on hold, he said.
The council still has a separate £2m reserve for emergencies, he added.