“Brexit will naturally have significant implications for the revenues of the European Union, and its assets and liabilities,” said Kevin Cardiff, Ireland’s member of the Court of Auditors.
“The implications for individual member states’ contributions have simply not been addressed yet,” said Mr Cardiff, who was secretary general of the Department of Finance, when Ireland went into its EU-IMF bailout programme, in 2010.
“We’re certainly well-aware these issues are coming down the track,” he added.
In total, UK contributions made up 16% of the EU’s €145bn budget last year, the second-highest, behind Germany, at 20%. Ireland’s contributions made up 1.3% of the total.
The UK has indicated it will trigger the start of its EU exit before March 2017.
The UK will continue to pay into the EU budget and receive subsidies, though it pays in much more than it receives, a net amount of around €10bn in 2015.
But when the exit process is completed, the UK may be required to make additional payments, on top of its ordinary budget contributions. According to auditors, the EU has liabilities and accrued costs amounting to €121bn, which it has not planned-for in future budgets.
Assuming a budget share of 16%, it could mean the UK is on the hook for around €20bn, in gross terms.
The president of the European Court of Auditors, Klaus-Heiner Lehne, said there was no way of knowing what the final Brexit bill will be for either side, as it’s not clear what kind of deal the UK will end up negotiating with the EU.
“It’s a black hole,” Mr Lehne said, adding that the EU and UK positions now are “so far away” from each other that a ‘divorce’ settlement might not even be completed within two years, the legal time-limit set in the EU treaty.
British contributions to the budget, post-Brexit, will depend on whether the country chooses to pay to retain access to the EU’s single market, as Norway and Switzerland do, though the UK government has made clear that it will not accept paying close to its current EU budget commitments.
Once the UK leaves the EU, the bloc will have to decide whether to cut the budget or seek increased payments.