Chancellor George Osborne is facing mounting pressure to explain what he knew and when about a shock demand for Britain to pay Brussels an extra £1.7 billion.
David Cameron responded furiously to the bill from the EU yesterday, complaining that he had been ambushed at a summit of fellow leaders.
Warning that the move risked pushing the UK closer to the exit door, the Prime Minister insisted the money would not be paid by the December 1 deadline.
But the European Commission has dismissed the objections, saying the contribution revisions were calculated by independent statisticians using a standard formula agreed by all member states. That process varies the fees charged depending on economic performance.
Britain has by far the biggest extra demand, with other countries such as the Netherlands, Italy and even crisis-hit Greece also facing paying more. France and Germany, on the other hand, have had their contributions cut.
Labour has seized on news that the Treasury was notified of the extra bill days before Cameron was informed. They also questioned why ministers failed to realise there was a potential issue earlier in the calculation process - pointing to recent upwards revisions of post-1995 Gross National Income by the UK's own statistics watchdog.
In a letter to the Chancellor, shadow chief secretary to the Treasury Chris Leslie wrote: "Following the European Council this week and the concern over the increased contribution from the UK Government to the European Union Budget you have serious questions to answer about how long the UK Government has known about the possibility of a higher surcharge for the UK.
"The Office for National Statistics published a report over four months ago detailing the changes made to UK growth figures which it stated were for use in the calculation of a Member State's contribution to the EU Budget.
"These surcharges are the conclusion of a process launched in 2011 by the European Statistics agency Eurostat, and cited by the ONS in 2012, which was designed to harmonise the GNI calculations for EU nations."
Leslie demanded to know when Treasury officials first became aware that the process could lead to a higher bill.
He also asked: "When were you made aware of the possibility of a surcharge for the UK emerging from this process?
"Did you or ministers raise with officials either at the ONS, the Treasury, or FCO any concerns about this emerging issue?
"Since the new GNI statistics for 1995 to 2014 were published in May what steps has the Government taken to raise the issue of this potential surcharge with EU partners or the Commission?
"At what point did you notify the Prime Minister, in the week of a European Council meeting, of these proposed surcharges?
"Why did you fail to make these surcharges public when you were notified of them?"
Leslie questioned whether ministers came to parliament without giving information about the issue, and insisted full details of the calculations should be published when they obtained them from the EU.
"The public deserve answers, not just posturing, on a matter which could see billions of pounds of taxpayers money lost," he added.