Jonny Wilkinson has admitted there is ''a challenging effect'' created by the balls being used at the Rugby World Cup in France.

New Zealand fly-half and goal-kicker Dan Carter raised concerns that balls players have been given in practice are different from those used in matches.

World Cup organisers have asked ball manufacturers Gilbert to examine Carter's comments, which came after he missed five out of nine kicks during the All Blacks' victory over Scotland at Murrayfield last Sunday.

No official complaints have been made by the New Zealand camp, but Gilbert are conducting their own inquiry.

Wilkinson, despite booting 24 points in England's win against Samoa, also experienced a couple of uncharacteristic misses.

Wilkinson, England's record points scorer, said: ''I know there have been some guidelines put in place in terms of the development of the balls.

''People are trying to create balls that are perhaps easier for handling in the wet, or when you play night-time rugby and there is dew on the ball, but also on very dry days when your hands are very sweaty, which can be a lot worse.

''It has brought up issues with kicking the ball since the World Cup warm-up games (in August) and it has given us a new challenge certainly.

''I don't care if I am kicking badly, as long as I know it is me and I can work on it.

''The difficult thing is if you are kicking well, and you ask yourself, 'Is it me or not?' That is what affects my concentration.

''I missed a couple of kicks against Samoa which were very heavy.

"Both shifted quite a way, right to left, which with no wind is quite unusual for me.

''The next kick, you are then asking, 'Do I allow for that, treat it as a one-off or do I ignore it?' That is the tough part, you end up playing mental games outside a game, which you don't really want to be doing.''

Wilkinson will again have a key role to play with the boot when England target a World Cup quarter-final place in Friday night's tournament eliminator against Tonga at Parc des Princes.

He is closing in on 950 international points, and as ever his preparation will be meticulous.

Wilkinson added: ''I try to select the balls I practise with very carefully and stick to the same ones, and I have been kicking better in practice than I did against Samoa.

''After the game, I just wanted to get straight back out there and thought, 'Let's work on it again and find a way to beat it.'

''It is maybe just a time issue, maybe just a case of getting used to them.

''It is a challenging effect of the new balls, when you have been used to certain things happening for a long time, but I haven't had time to work with them for so long.

''In situations where three points really matter, it is a huge amount of pressure.

''But there are some great kickers out there who have shown they can kick the ball very well, and everyone is in the same boat.''