Sebastian Vettel has Mercedes 'pre-agreement'?

Sebastian Vettel may already have a "pre-agreement" to switch to Mercedes for 2018, according to Formula 1 media insider Leo Turrini.

Although leading the world championship, German Vettel - whose Ferrari contract expires late this year - is rumoured to be in talks with Mercedes.

"It is no secret to anyone that after Rosberg's retirement, Mercedes contacted Seb," wrote Turrini in his Quotidiano blog.

"During the pre-season test in Barcelona, Niki Lauda told friends confidentially that Vettel has a pre-agreement with the Germans for next season."

However, Turrini said that Ferrari subsequently surprised with their strong pace, and therefore could ink a new deal with the quadruple world champion for 2018 and beyond.