Wilkinson, 30, was last week included as one of only two dedicated fly-halves in Johnson's 32-man squad despite not having played for 10 months after suffering a dislocated knee-cap.

Danny Cipriani was among those overlooked and Johnson has told the London Wasps fly-half he will only force his way back into the senior England team on form.

But Johnson made no apologies for including Wilkinson, his 2003 World Cup-winning team-mate, who has returned to full training with his new club Toulon.

"How do I justify picking Jonny Wilkinson? Seventy caps, two World Cup finals and x amount of points (1032 in total) - the sort of player you are going to favour slightly! He is a world-class player when he's fit," said Johnson.

"I think he can be as good as he ever was. Experience is a big part of being a good player and he's only 30 so potentially he could be at his peak because of his experience and what he's been through on and off the field over the last five, six, seven years."

Wilkinson's career has been ravaged by a catalogue of injuries and ailments since he teamed up with Johnson to help England win the World Cup in 2003.

England's rugby director Rob Andrew believes the dislocated knee suffered playing for Newcastle last October was the "cruellest" of them all because of Wilkinson's eye-catching form in the preceeding months.

But Wilkinson will be fit to start the season with Toulon and the RFU medics were impressed with his condition after undergoing a fitness assesment at Twickenham.

Johnson continued: "The guy, in my experience, has worked as hard as anyone on his fitness and his body.

"Jonny was in here last Friday for a fitness assessment and he is in good nick. I spoke to him yesterday and he's training fully down there (in Toulon) and he's raring to go."

Cipriani was annointed as Wilkinson's heir apparent after replacing him in the side for England's 2008 RBS 6 Nations victory over Ireland at Twickenham.

But Cipriani struggled to rediscover form after suffering an horrific fractured dislocation of the ankle and he finished last season on the bench for the second-string England Saxons behind Northampton's Stephen Myler.

Johnson insisted Cipriani's continued omission from the senior squad was down to form alone.

"Danny is just one of a number of young players who've had a taste of it, won a couple of caps and are now battling to get back into a competitive squad," said Johnson.

"He's a talented young player. He's got areas of real strength and areas that he needs to improve, which is the same for most young players. He just has to work on those areas of weakness.

"That is what being a rugby player is about. If it was all easy all the time everyone could do it. We want guys who are desperate to play for England. These guys say they are and I am sure they are - they have to show us with their form."