Britain's departure from the EU will leave a €10bn black hole in the European Union budget that will pit rich and poor EU members states against each other in a fight over how to fund the Union, a leading EU think-tank has warned.

The warnings over the so-called "Brexit Gap" came as Theresa May prepared to deliver her keynote Brexit speech in which she urged the European Union not to see the UK's decision to quit the Union as a hostile act.

“Our vote to leave the European Union was no rejection of the values we share," Mrs May said, "The decision to leave the EU represents no desire to become more distant to you, our friends and neighbours."

However the report by the Jacques Delors Institut warns of bitter struggles to come among EU members as they seek to heal the financial shortfall left by the departure of Britain which makes a net contribution to the EU of around £8.5bn (€10bn) a year.

"It is clear that Brexit will deal a shock to the EU budget. There is no easy way to fill the 'Brexit Gap' of around €10bn per year," the report said.