Estonia and Poland’s Prime Ministers have spoken out against Britons campaigning to leave the EU, ahead of a nationwide referendum on the issue later this month.

The referendum will ask British citizens to cast their ballot to decide whether or not the U.K. should exit the EU after 43 years as a member. The ‘Brexit’ debate has split the U.K.’s ruling Conservative Party, with Prime Minister David Cameron campaigning to remain in the EU but senior officials and former allies in the House of Commons favouring an exit.

During a meeting in Warsaw, Poland’s Prime Minister Beata Szydlo and her Estonian counterpart Taavi Roivas issued a joint plea for the U.K. to remain in the EU, Polish public broadcaster Polskie Radio reports.

“We agreed that we very much want Great Britain to remain in the European Union,” she said. “That is very important for the European Union, for its durability, for its stability, as well as for the economic prospects of both our countries.”

Previously, Poland’s President Andrzej Duda warned that a Brexit could result in the collapse of the EU, while Deputy Foreign Minister Konrad Szymanski said there would likely be no large-scale exodus of Polish workers in the case of a Brexit, despite campaign claims.