EU countries need to reach a swift agreement on the bloc's next budget after the loss of Britain and its hefty financial contribution, 15 member states agreed in Portugal on Saturday.

The "Friends of Cohesion" group of 17 eastern and Mediterranean countries, including Malta, also agreed to fight any move to cut the EU Cohesion Fund reserved for the bloc's weaker economies.

"More than ever we have to affirm the cohesion of the European Union, as it is now more fragile with the departure of the United Kingdom," Portuguese Prime Minister Antonio Costa told journalists in Beja, southern Portugal.

Fifteen countries agreed to defend the Cohesion Fund. 

Bulgaria, the Czech Republic, Cyprus, Estonia, Greece, Hungary, Latvia, Lithuania, Malta, Poland, Portugal, Romania, Slovakia, Slovenia and Spain all signed the closing statement.

Italy chose not to sign. Croatia was the other non-signatory, choosing to remain neutral as it holds the rotating EU presidency.

Charles Michel, president of the European Council which represents the 27 member state governments, has called a special budget summit in three weeks' time to agree the bloc's budget for 2021-2027.

Members are divided on the budget, and some observers fear the talks may drag on for days.

"It's urgent to reach an agreement on the next European budget and the member states who met today made it clear they want to reach an definitive agreement on February 20," Costa said.

With Britain leaving, so is its 12-billion-euro (13-billion) contribution to EU finances -- even if it will keep making contributions this year under the agreed post-Brexit transition period.

The Cohesion Fund is for member state whose per-capita gross national income (GNI) is less than 90 percent of the EU average.