Ben Spies has explained what triggered his shock decision to leave the Factory Yamaha MotoGP team.

Spies had been a Yamaha rider since switching from the AMA to WSBK - with instant title success - in 2009 and won his first MotoGP race during a successful 2011 debut season at Yamaha's official grand prix team.

2012 saw a relentless run of incidents, accidents and bad luck - but it was still a major surprise when Spies announced, halfway through the season and via the internet, that he would not ride for the team in 2013.

The Texan subsequently revealed behind the scenes tension and has now confirmed that comments made at Mugello by a 'senior Yamaha employee' were behind his decision to walk away from the title winning M1.

"I had food poisoning at Mugello [the Italian Grand Prix]. Bad food poisoning. I shouldn't even have started the race. I got sick in my helmet, and afterward, I was dry-heaving and shaking uncontrollably," Spies told

"Yamaha stayed in Italy and tested the next day, but I didn't ride. I couldn't ride. I couldn't move. I couldn't do anything.

"A senior Yamaha employee - that's as specific as I'm going to get - said to me, 'We've invested a lot of money in you. Don't come to Laguna Seca if you aren't 100 percent.'

"Then, he added, 'We've lost confidence in you.'

"That was the moment when I decided I wasn't going to ride for Yamaha in 2013.

"I have a lot of good friends at Yamaha, but when someone talks down to you like that, you lose respect for them."

Spies - whose place alongside Jorge Lorenzo has since been taken by Valentino Rossi - admitted that he seriously considered a return to WSBK, with BMW, but ultimately decided to stay in MotoGP.

Despite interest from Honda, the #11 will ride for the satellite Pramac Ducati team, where he will have "factory bikes, factory everything".

"The Yamaha is a great bike, but it likes a lot of corner speed," Spies said of the M1. "If it moves, if the wheels get out of line, it gets upset.

"Both the Honda and the Ducati look like they can be ridden a little more 'wild'."

Having been denied the chance to make his Ducati debut at the Valencia test due to a season-ending shoulder injury at the Malaysian GP, Spies will ride a Desmosedici for the first time at the Sepang test in early February.