Red Bull has denied claims the struggling world champions are secretly running the troubled Renault-powered RB10 between the two Bahrain tests.
Spaniard Pius Gasso, a respected and reliable F1 source and former racing driver, said he believes an RB10 is running at the Idiada facility in Spain, south of Barcelona, ahead of the resumption of official testing later this week in Bahrain.
Gasso even 'tweeted' a link to an audio file of the alleged 'secret' testing.
Another source told Speed Week correspondent Matthias Brunner that Red Bull's Idiada running would be contrary to the tight testing limitations.
"They would only be able to drive at Idiada with a current car if they had declared it a 'PR day'. And the rules say clearly that it would be purely for marketing and PR purposes," said the F1 source.
A Red Bull spokeswoman said simply: "We are not at Idiada!"