Green members of the European parliament will not support Ursula von der Leyen for a second term as the commission president if she makes a deal with hard-right nationalists, the party’s joint lead candidate has said.

Terry Reintke, the German Green MEP chair, said her group would “absolutely” not support von der Leyen – the incumbent centre-right commission president who is seeking a second term – if she made a deal with the Italian prime minister Giorgia Meloni’s group in the European parliament, the European Conservatives and Reformists (ECR).

“We are not going to join the coalition or the majority, for example, with the ECR. So there, Ursula von der Leyen, or whoever wants to build a majority, will have to make a choice,” Reintke said.

Reintke warned that if von der Leyen joined forces with the ECR, which has repeatedly voted against EU green policies, the EU’s plan to tackle the climate crisis would be in danger. “It’s much more likely that the green deal will be killed, or at least slowed down”. She said this would be “a disaster not only for the climate, but also for the economic standing of Europe” in the face of intense competition from the US and China.

The intervention deepens the dilemma for von der Leyen, the clear frontrunner for the commission top job, after a similar pledge from the centre left, which has said it will not support her if she works with hard-right parties, including Meloni’s Brothers of Italy.