According to an unnamed European official talking to WirtschaftsWoche, Britain won’t be able to leave the bloc until it pays off €25 billion: “A deal with Great Britain is unthinkable until the Brits settle this liability.”
Under EU regulations, each bloc state contributes funds to a joint account that is used for realization of large-scale infrastructural projects within the bloc through the years. In practice that means that if the EU funds a bridge, the full sum spent on it will be returned to the joint budget only when project is completed.
The European Commission official didn’t specify how much money the UK owes to the joint account, saying it’s not easy to calculate each state’s required contribution.
Moreover, the source clarified that “outstanding commitments are not debt. No member state has outstanding debts to the EU budget.”
Still the payments should be processed, if London decides to follow through with Brexit: “The Commission will not speculate about any implications on any future negotiations when Article 50 has not even been triggered.”
"At every step, we will work to ensure the best possible deal for the British people. We are about to begin these negotiations and it would be wrong to set out unilateral positions in advance. Brexit means Brexit and we are going to make a success of it.”
According to the Guardian, the UK allocates £136 million weekly to the joint European budget. The country is expected to pay a total of £45.3 billion to the bloc in the period 2016-2020.