MotoGP World Championship leaders Valentino Rossi and Jorge Lorenzo have welcomed the 'early' arrival of Ben Spies to grand prix racing for the 2010 season.

On the eve of this weekend's Portuguese Grand Prix it was confirmed that the 2009 WSBK rookie and title contender will switch to MotoGP next season, replacing James Toseland at the satellite Tech 3 Yamaha team.

Rossi, who claimed he only knew of Spies' decision when it was publically announced on Thursday, said the move made sense for the Texan.

"I expected that Ben would make another season in Superbike, but I understand because his target is to arrive in MotoGP," said Rossi, the reigning six time MotoGP champion.

"Also he has the chance to win the WSBK championship already this year, because he is very close to Nori [Haga].

"I think it is the right decision to come straight away. He is quite young and he will now have one extra year to gain experience and to understand MotoGP. He will be another great rival for next season.

"I think Spies has the potential, but for sure he will need time to understand the bike and the tyres and he will have to demonstrate that he can be a top MotoGP rider," clarified Rossi. "Anyway, I think he will be good for Yamaha's future and it is good for MotoGP.

"We are not a lot, but the quality is very high. So the number [of riders in MotoGP] is not fantastic but with Ben all the best motorcycle riders in the world will be racing in MotoGP so it is more interesting."

But Rossi warned that Spies faces a tougher task than those entering MotoGP from 250cc.

"For me MotoGP riding style is very close to 250 riding style now and riders like Jorge, but also Dani [Pedrosa], Stoner and maybe Simoncelli next year - arriving from 250 - have a big advantage over the riders coming from Superbike," he said.

"Because the riding style with Bridgestone tyres is more similar to 250 and a big difference from Superbike. Also the 250 riders know nearly all the tracks and World Superbike riders only two or three of them.

"So I think it is a lot more difficult for World Superbike riders to arrive at the top in MotoGP, at the beginning, compared to rider from 250 like Jorge - who took three poles in the first three races last year.

"But maybe it is just a matter of time, because Ben is young and I think he is the only guy from Superbike who is able to try and win in MotoGP at this moment."

Rossi starts the final four rounds of the 2009 season with a 30 point lead over factory Yamaha team-mate Lorenzo, who also thinks highly of Spies.

"It is very good news for Yamaha to have another very strong rider," said the Spaniard, the only rider other than Rossi to win an 800cc grand prix for Yamaha. "He is one of the best riders at the moment in Superbike. I think he is a strong character and I think he can do great things in MotoGP."

Yamaha's full 2010 MotoGP line-up is thus confirmed as Rossi and Lorenzo at Fiat Yamaha, plus Colin Edwards and Spies at Monster Yamaha Tech 3.

Triple AMA Superbike champion Spies - who made his MotoGP debut with Rizla Suzuki last season and claimed a best race finish of sixth - starts the final two rounds of the 2009 World Superbike Championship just three points behind Haga (Ducati).