Rossi tests his Yamaha at Jerez, where he won last season's Spanish GP

The 2006 campaign opens with the Spanish Grand Prix on 26 March but Rossi could only finish 10th fastest in the final pre-season test in Jerez.

The Italian's machine has developed persistent vibration problems and Rossi told Motorcycle News: "We are in a lot of trouble for the first race.

"This has been the worst test of the winter and I'm quite worried."

The 26-year-old winner of the last five MotoGP titles was more than a second off the pace in testing set by compatriot Loris Capirossi on a Ducati.

His American team-mate Colin Edwards, on the other Yamaha, was seventh.

"At this point we don't know which way to follow because we have tried everything," Rossi said.

We were always fast but with this problem we are struggling Valentino Rossi

"New settings, new tyres, different forks, different shocks - but the vibrations remains strong.

"It's bad, because we worked hard during the winter and the bike was always good.

"We were always fast, but now with this problem we are struggling."

However, Rossi's bike worries were not enough to prevent him having a dig at Formula One world champion Fernando Alonso.

Alonso unimpressed by Rossi tests

The Spanish Renault driver said last month following Rossi's recent tests for Ferrari that it would be "very difficult" for the Italian to be competitive if he switched to four wheels.

Rossi responded: "I read that if Alonso is able to make some practice he can put a MotoGP bike on the podium with me.