Jenson Button has branded his F1 rival and countryman Lewis Hamilton "very weak" for pulling out of a triathlon challenge for charity.

After declaring last week he is fitter than Button, Hamilton was pressured into going head-to-head with the Honda racer in the Thursday press conference at Silverstone -- before his father Anthony pulled the plug less than half an hour later.

"That's weak, very weak," Button is quoted as saying by the Swiss publication Motorsport Aktuell.

"Honestly, I'm surprised. He's an athlete, I'm an athlete, we both love the challenge."

When Hamilton originally accepted the challenge, Button said he would donate $20,000 to a charity if he is beaten.

"What happens to the money now?" Button continued. "Hopefully he still donates it ..."

Source: GMM