Apr.15 (GMM) F1 drivers are threatening to strike if their teams do not pay them.

Germany's Sport Bild reports that, in Bahrain recently, Kimi Raikkonen's manager Steve Robertson was seen arguing in the paddock with Lotus team boss Federico Gastaldi.

Reportedly, that is because the Finn, who switched to Ferrari at the end of last year, still has not been paid in full by Lotus for the 2013 season.

The magazine claims that Romain Grosjean, Nico Hulkenberg, Adrian Sutil and Kamui Kobayashi are all also waiting on overdue payments.

It is believed that, under the auspices of their union, the GPDA, the F1 drivers have signed a document vowing to strike if the growing trend of not being paid continues.

Sport Bild claims that Raikkonen - not a GPDA member - and one of F1's very highest earners, Lewis Hamilton, have refused to sign the document.

Hulkenberg would not comment.

"What we talk about in meetings, we keep to ourselves," said the German, who moved from Sauber to Force India for the 2014 season.

He admitted, however, that unpaid wages is an issue for the drivers.

"That's right. The teams are aware of the situation -- the driver can be easily replaced. Maybe not with the same quality, but they (the teams) do take advantage of that."

Hulkenberg also expressed some understanding for the situation of the teams.

"They are not doing it (failing to pay drivers) for fun," he acknowledged. "The money just isn't there. The sport is too expensive."