No 2015 upgrade for Mercedes teams

All Mercedes-engined drivers are due to fit their fourth allocated units of 2015 for this weekend's Russian Grand Prix, apart from Nico Rosberg and Nico Hulkenberg, who are already using their fourth power units.

Championship leader Lewis Hamilton will use a fresh version of the 'phase four' development engine introduced on the works Mercedes cars at September's Italian GP.

The German manufacturer spent its remaining seven 2015 development tokens on a combustion upgrade for Italy, as it explored what it called a "new development direction".

But the Williams and Lotus drivers, plus Force India's Sergio Perez, will continue using the previous iteration for this weekend's Sochi race and the remainder of the season.

Williams said at Monza it hoped to tie its final scheduled engine changes of 2015 in with an update from Mercedes, but AUTOSPORT understands the German marque is unable to produce enough of the latest specification of parts to supply all of its customer teams this year.

The customer teams will now have to wait until 2016 before they get the opportunity to update their power units again.