The president of the European Commission has warned against believing claims that Brexit will not happen and that Britain will remain in the EU.

Speaking at a discussion of the next EU budget in Brussels Jean-Claude Juncker said the Commission’s “working hypothesis” would be that “our British friends will be leaving us” and that future spending had to be planned accordingly.

Mr Juncker said the loss of the UK, a net contributor, from the EU budget, meant other EU countries would have to contribute more.

“We need more than 1 per cent [going to the EU budget] if we are to pursue EU policies and fund them adequately,” the Commission President said.

“Don’t believe those who say [Brexit] is not going to happen. Our working hypothesis is that our British friends will be leaving us.

“Between now and then we need to find the means of reacting to the loss of a significant number of billion of euros when a net contributor goes.”

Mr Juncker ruled out “drastic” cuts to the Common Agricultural Policy or EU cohesion funds, which he said were necessary to maintain Europe’s food security and to even out disparities in wealth between different parts of the union.

Justifying increased spending, he said EU membership was worth roughly “one cup of coffee a day” for the EU taxpayer, adding: “I think Europe is worth more than one cup of coffee a day.”

The current budget round started in 2014 and will end in 2020, and the EU will have to agree a new budget before then.

The EU’s current budget is around €105 billion a year, or £92.8 billion. The largest gross contributors ahead of the UK are Germany, France, and Italy, who contribute around 21 per cent, 16 per cent, and 14 per cent respectively. The UK contributes 13 per cent.

The UK had the second highest net contribution, however, behind Germany. Only nine of the 28 member states are net contributors to the redistributive budget.

Blair urges Labour to challenge Brexit

As part of ongoing Brexit negotiations, the European Parliament has called for the UK to make “commensurate” payments to the EU for access to the bloc’s markets – though UK figures, including Foreign Secretary Boris Johnson, have ruled this out.

Under the understanding reached by negotiators in December, the UK will continue to contribute to the EU budget until the end of the current spending round in 2020.

Brexit: the deciders Show all 8 1 /8 Brexit: the deciders Brexit: the deciders European Union's chief Brexit negotiator, Michel Barnier Getty Brexit: the deciders French President Emmanuel Macron Getty Brexit: the deciders German Chancellor Angela Merkel Reuters Brexit: the deciders Commission President Jean-Claude Juncker EPA Brexit: the deciders The European Parliament's chief Brexit negotiator Guy Verhofstadt Getty Brexit: the deciders Britain's Prime Minister Theresa May Getty Images Brexit: the deciders Britain's Chancellor of the Exchequer, Philip Hammond PA Brexit: the deciders After the first and second appointed Brexit secretaries resigned (David Davis and Dominic Raab respectively), Stephen Barclay is currently heading up the position PA

Eloise Todd, chief executive of Remain campaigners Best for Britain, said: "President Juncker is completely wrong to say Brexit is a dead certainty.

"We have until March 2019; the government wants the date set in stone because every day that goes by exposes their strategy of pretending Brexit is all things to all comers.