Britain's exit from the European Union will mean a worrying 15 percent drop in the bloc's annual budget from 2020, EU Regional Policy Commissioner Corina Cretu said on Monday.

EU leaders are due to discuss the outcome of Britain's June 23 vote to leave the EU at a meeting in Brussels on Tuesday. Britain is a net contributor to the EU budget, much of which goes to support farmers and poorer regions around the bloc.

"Regional policy is the closest to the people. It is the most visible, as it shows how European funds change their lives, so from this point of view I'm worried because it's clear that Britain's exit will make the overall EU budget after 2020 fall by about 15 percent," Cretu told Digi24 TV from Brussels.

Cretu, who is Romania's commissioner, also said the EU was engaged in "full talks over fund re-allotments for other priorities than regional policy" post-Brexit, adding: "We need to return to those values that formed the basis of the EU, with responsibility and solidarity from everybody."

"At the end of the year we'll have a revision of the multi-annual financial framework. I believe there will be voices, especially from big contributing states who would want to re-allot, though I don't think this is possible for this period."