The leader of Germany’s Social Democrats has called on EU members to commit themselves to a “United States of Europe” by 2025, setting out an ambitious European reform agenda as his party agreed to talks with Angela Merkel on the formation of a new -government.

Martin Schulz told a party conference in Berlin yesterday that he wanted EU member states to agree a new “constitutional treaty” to establish a federal union and that countries that did not sign up would have to leave the bloc.

“Such a constitutional treaty has to be written by a convention that includes civil society and the people. This constitutional treaty will then have to be put to the member states and those that don’t approve it will automatically have to leave the EU,” Mr Schulz said. A previous attempt at giving the EU a new constitution more than 10 years ago collapsed after its rejection in referendums in the Netherlands and France.