An agreement that paves the way for Angela Merkel to form a new coalition government in Germany has been cast in doubt by a growing rebellion in the Social Democrat party (SPD).

Mrs Merkel and the SPD leader, Martin Schulz, on Friday sealed a deal after marathon 24-hour talks to open formal negotiations on renewing their coalition.

But the agreement has to be approved by a special SPD party congress next Sunday, and there are signs of growing opposition in the party, which suffered heavy losses in last September’s elections.

“There was a clear vote against both coalition partners,” Michael Müller, the mayor of Berlin, told Tagesspiegel newspaper.

“The same coalition with the same policy is not a good enough answer to this. A continuation without decisive changes does not convince me yet.”

If the SPD votes against joining a coalition, it would leave Mrs Merkel facing the prospect of new elections or trying to form a minority government.

Her agreement with Mr Schulz has already failed its first test, after the SPD regional association in the state Saxony-Anhalt voted to oppose a new coalition this weekend.

The small state only sends seven delegates to the SPD congress out of a total of 600. But the decision has sent jitters through the SPD and the party leadership is scurrying to shore up its support.