As the Prime Minister stood up in parliament to proclaim Britain "will not be paying" the €2bn bill that landed on his doorstep last week, a senior member of the European Commission was warning the British government that not to pay would be to "open a Pandora's box".
Jacek Dominik, the EU's budget commissioner, told a press conference in Brussels that a refusal to pay will put the UK's annual rebate at risk.
Never in the past was there a situation where such a decision was changed and the implementation regulations changed because one of the member states contested it. I am afraid that it will be extremely difficult to do it, especially because the own resources decision and implementation regulations concern as well the UK's rebate, so if you open this for future negotiations, you open a Pandora's box.
In a session in parliament today, David Cameron attacked the "scale and timing" of the bill. But Dominik pointed out that officials had been warned of the size of the UK's bill as long ago as 17 October.
He added that he was "surprised" that the British government had reacted so strongly, given that the calculations used by the EC mean the UK is likely to receive a £500m increase in its annual rebate - which is usually around £3bn - next year.