The UK has been told to contribute an extra £1.7bn (€2.1bn) to the European Union's budget after the economy performed better than expected.
The EU is asking for the payment after a revised calculation of how much each member state must contribute, based on gross national incomes, the BBC reports.
The payment adds a fifth to the UK's annual net contribution of £8.6bn. It is based on a review of the UK's economic performance since 1995, and the pace of growth over the last four years.
A European Commission spokesman described the payment as a result of the UK's "increase in wealth" and likened it to a tax payment.
But the additional payment comes at a time when the government is under pressure from eurosceptics. Earlier this month, Conservative defector Douglas Carswell won a by-election for anti-EU party UKIP in Clacton. Some fund managers have already outlined concerns of political risk as the election looms.
Prime minister David Cameron is meeting other EU leaders for a summit in Brussels. A government source told the BBC: "It is not acceptable to just change the fees for previous years and demand them back at a moment's notice.
"The European Commission was not expecting this money and does not need this money and we will work with other countries similarly affected to do all we can to challenge this."
