The man who could be Germany’s vice-chancellor within weeks on Thursday called for the European Union to transform itself into a “United States of Europe”.

Martin Schulz, the leader of the German Social Democratic Party (SPD), called for a new federal constitution for the EU by 2025.

Hours before his party voted to open talks on forming a new coalition with the beleaguered Angela Merkel, Mr Schulz made clear he would demand radical EU reform and far deeper integration than previously envisaged as his price for ending weeks of political crisis in Germany.

“I want a new constitutional treaty to establish the United States of Europe. A Europe that is no threat to its member states, but a beneficial addition,” he said in a speech to his party conference.

Under Mr Schulz’s proposals, Brussels would be given power over individual member states’ foreign and domestic policy, as well as taxes.

Countries who refuse to sign up to a new federal Europe should automatically lose their EU membership, he said.

A deal with Mr Schulz’s party is Mrs Merkel’s last chance to prevent new elections after coalition negotiations with smaller parties broke down last month.