WARSAW — Radek Sikorski, a former Polish foreign minister with long ties to Britain, says he is ‘‘very worried’’ British voters could opt to leave the European Union, fearing it could weaken a political arrangement that has given Europe its longest-ever period of peace and prosperity.

Sikorski said Monday that a weaker EU would be particularly risky for nations like his, which has historically been at the mercy of its large neighbors Russia and Germany. He also argued that it threatens to leave Britain more isolated and economically weaker.

‘‘I think Britain’s membership has been hugely beneficial to the people of Britain,’’ Sikorski said. ‘‘So I hope they stay with us. We would miss them.’’

Britain is holding a referendum on June 23 to decide whether to leave the EU or remain. The possibility of Brexit, as a British departure from the EU is called, poses a major existential crisis to the 28-member bloc, which is also being sorely tested by the migration crisis.

Associated Press