Multiple world champion Alain Prost has poured oil on the waters of Michael Schumacher's decision not to return to Formula One at next weekend's European Grand Prix, suspecting that the German's neck injury might have been the tip of a bigger fitness iceberg.

The Frenchman has claimed that Schumacher's shock about-turn may have less to do with discomfort from an injury sustained while testing for a prospective season of motorcycle racing as it has with the realisation that coming back to the top flight after a near three-year lay-off would be tougher than he initially imagined.

Prost is among an elite group of drivers to have taken time out of F1 and returned with success - having sat out the 1992 campaign after being fired by the Scuderia and then returned to take the following season's world title with Williams - but he fears that Schumacher was attempting too much following his 2006 retirement, especially pitting himself against younger, maybe fitter, rivals on the Valencia street circuit.

"It remains to be seen whether he only called his comeback off because of health problems," the former Ferrari pilot told France's Parisien newspaper, "Perhaps, when he climbed back into the cockpit, he realised that there was an enormous task ahead of him. The body changes very fast when one stops driving [in F1], the reactions and visual acuity are not as sharp. He has not driven an F1 car for three years, and had only three weeks of preparation [following Felipe Massa's injury]."

Prost accepts, however, why Schumacher had been tempted to return to the fray, and claims that the German was probably right to ultimately decide against coming out of retirement in Valencia.

"The desire and motivation to return takes time to disappear, it takes several years to abandon F1," the Frenchman admitted, "Michael and I hung up our helmets for different reasons, but when you were competitive the temptation to return when asked is great.

"If there was any physical risk, Schumacher was right [to change his mind]. The neck is crucial in motorsport and, if there is pain, you can quickly feel nausea and have impaired vision. When I returned in 1993, after eight months away, it was very difficult to find the level, and Schumacher may need more time."

Schumacher's manager Willi Weber was quick to dismiss claims that the German had flinched at the thought that he may not have been competitive, and confirmed that his charge may continue to test with the Scuderia through to the end of the season in case a belated comeback is possible in the six races that follow Valencia. Schumacher, it was pointed out, had only been allowed to test in a 2007-spec Ferrari, albeit fitted with slicks rather than the grooved Bridgestones of its day, and could not compare his performance to anyone else.

"Unfortunately, I must disappoint Mr Prost," the German retorted via the German press, "Michael's times in testing at Mugello suggested different - his decision to call off his return was due only to the neck problem. He can now continue to work on his fitness in peace, without the pressure to prove anything. Perhaps then we will see...."

Schumacher's spokeswoman, Sabine Kehm, confirmed that an F1 comeback later in the season is not being ruled out but, while the German may return to the track in an F1 machine, his father has revealed that any plans the seven-time world champion may have had of switching his abilities to two wheels were now officially on hold. Schumacher Jr had not enjoyed the smoothest of transitions, with the media happy to report every spill he took during the early days of his new career.

"Michael has said to me that he will do no more motorcycle racing," Rolf Schumacher confirmed to Bild-Zeitung, reflecting on the 100km/h+ crash in testing at Cartagena that left his son with neck and skull fractures, "That is good news for me as motorcycle racing is so much more dangerous than F1. He will still ride, but it will be more as a hobby and for training."