European Union leaders ended an acrimonious summit with Russia on Friday by pledging to stand together in the face of Kremlin pressure on Poland.

José-Manuel Barroso, the European Commission president, warned Moscow that if it wanted close co-operation with the EU it was “very important” to understand that the union was “based on principles of solidarity”. A difficulty of one member state was shared by all members. “The Polish problem is a European problem. The Lithuanian and Estonian problems are also EU problems,” he said.