The big rumour doing the rounds in Sochi is that McLaren is to use Mercedes power units from 2021

Though neither company has confirmed - or denied - the claim, a number of major news outlets insist that it is true.

McLaren first partnered with Mercedes in 1995, following a one-season deal with Ford and another with Peugeot, while the German manufacturer had returned to the sport the previous year with Sauber.

Despite a slow start to the partnership, which saw McLaren finish fourth in the standings for three consecutive seasons, Mika Hakkinen won back-to-back titles in 1998 and 1999.

Through the years of Michael Schumacher/Ferrari domination, McLaren-Mercedes continued to be a major force, and just a year after being excluded from the championship following the Spygate scandal, claimed the title once again this time with Lewis Hamilton.

In 2010 however, Mercedes bought the title winning Brawn team and returned to the sport as a constructor, even luring Michael Schumacher out of retirement.

From the outset of the hybrid era, Mercedes was clearly the benchmark, but with McLaren now merely a customer of the manufacturer rather than a fully-fledged partner, the Woking outfit opted to join forces with another manufacturer with which it had enjoyed great success, Honda.

The decision to switch back to Mercedes makes sense.

Other than continued concerns over Renault's reliability, not to mention the fact that the French manufacturer still hasn't mastered the hybrid formula, even though it was a driving force in its introduction, there is concern at McLaren, as there was with previous partners, that in being an engine manufacturer and constructor, Renault has a conflict of interests, hence (team boss) Andreas Seidl's repeated calls for transparency.

Indeed, it is understood that Seidl is the man behind the decision, which would see McLaren powered by the best engine on the grid and at a better price than the Renault.

In the midst of its issues with Honda, McLaren is understood - much like Red Bull - to have sought a deal with Mercedes but was turned down.

If the switch does happen, it would mean Renault is only supplying one team... its own.

Check out our Friday gallery from Sochi, here.