Three-time world champion Sir Jackie Stewart says Nico Rosberg can win the title in 2016 if he can "step up that little bit extra" against Mercedes team-mate Lewis Hamilton.

Hamilton drew level with Stewart's British record of three championships by winning this year's F1 crown, which he wrapped up at the U.S. Grand Prix in late October. However, Rosberg finished the season on a high, claiming six straight pole positions and three race victories in a row.

Stewart says Rosberg must not be written off in 2016 but insists the German will need to raise his game across the whole season to beat the "aggressive" Hamilton.

"If Mercedes retain the leadership that you almost expect from Mercedes, I'm afraid you could be finding the same thing, but don't count out Rosberg," Stewart is quoted as saying by the Press Association when asked if Hamilton could win a fourth title next year. "There is nothing to stop Rosberg winning. He is quick. We have seen him have so many pole positions and he is smoother [than Hamilton]. I'd like to see him win the title. That would be very good.

"Lewis is quite aggressive so therefore he has won more races because of that aggression, but sometimes that can bite you also. Nico is good and there is no question about it. The trouble is you have to be very good and he has got to just step up that little bit extra."

Ferrari hopes to mount a season-long challenge against Mercedes next season, those Red Bull boss Christian Horner thinks the current regulations makes it likely the world champions will dominate again in 2016. Stewart echoes the concerns of Horner and thinks it will be difficult for anyone to get close next year.

"Mercedes will eventually be beaten, but I'm not sure the rest of the teams are really good enough yet to be able to take them on. More than a second a lap is too big a gap and that's not good racing.

"You never know, another year, you have an Adrian Newey suddenly coming up with something and the whole scene can change. So, you can't take anything for granted. But while Mercedes' money is there -- both in engine and their chassis -- it is going to be difficult for them to be beaten."