Max Verstappen has emerged as a shock contender to switch to Mercedes in 2020.

Dr Helmut Marko, the driver manager at Red Bull, has admitted to Auto Bild that Mercedes boss Toto Wolff regularly calls Verstappen and his father Jos.

"Toto Wolff calls him and his father repeatedly and has done for months," Marko said.

It is believed a Mercedes switch for Verstappen may actually be possible for 2020, even though the 21-year-old is under contract until 2021.

Auto Bild reports that a performance clause in Verstappen's contract would permit his departure in the event that Red Bull does not perform this year.

"I'm relaxed," Marko insisted. "The clause in Max's contract should be safe, and he feels comfortable with us.

"If we provide him with material capable of winning the world championship, I'm not worried that he will go."

However, so far in 2019, Red Bull has struggled. Both Marko and team boss Christian Horner admit that the Honda-powered car has an "aerodynamic problem".

"The tests last week were very good and very insightful," Marko said.

"We've pinpointed the problems of the race weekend and can now take concrete action. Basically, our car needs more downforce," he added.