Tensions between Porsche and Volkswagen are rising following a weekend report in Germany's Der Spiegel magazine that VW has given the sportscar company a deadline of today (29 June) to work out the terms of the merger of the two companies.

Porsche chairman Wolfgang Porsche  and dDeputy Uwe Hück said on Monday that the ultimatum by VW and the State of Lower Saxony was tantamount to blackmail.

They said in a joint statement: "We will not give in to such pressure or blackmail. Such action does not help anybody. It is detrimental to the entire cause. This is not the way to support and uphold common interests.

"We are planning a successful future together with VW to the benefit of both companies. Hence, we are deeply concerned and irritated by the wording of the ultimatum. The 21st century is not the time for ultimatums. We wonder what the whole matter is really about and whether the focus is still on our common cause at all.

"We sincerely hope that the perpetrators of the ultimatum, in consideration of our common interests, calm down again and follow up their proposals in internal discussions and not through headlines. We are open to such talks at any time."

Also today, Volkswagen publicly denied it had set such an ultimatum for agreeing on a merger deal. According to Der Spiegel, VW plans to take a 49.9% stake in Porsche before the Qatar Investment Authority acquires Porsche's Volkswagen shares.

This would mean 40% of a combined company would be owned by the Porsche and Piëch families, 15% by the Gulf state of Qatar and the state of Lower Saxony would retain 20%.

Porsche currently holds a 51% stake in VW although it has now abandoned its attempt to take complete control of the company after racking up some EUR9bn in debt while acquiring shares. In March it had to borrow EUR700m from VW.

Christian Wulff, the premier of the state of Lower Saxony said that Porsche SE must decide soon whether to integrate or not. He added: "We need to clarify now whether each will go his own way or whether there is a common solution."

He said that he felt a "certain irritation" after Porsche accused VW and Lower Saxony of issuing an ultimatum to agree to a deal by today. "Not all facts are known to everyone," he added.