Whitmarsh thinks Mercedes offered Hamilton ‘a bit more’ money
The McLaren chief answered "I don't know" when asked why he thinks the 2008 world champion decided to leave his boyhood team, but thinks a big factor is Hamilton's desire to spread his wings after six years with the famous British outfit.
But Whitmarsh told F1's official website that money was probably another factor.
"We made Lewis an offer — an offer which I believe is more money than any other driver at the moment is being paid," said Whitmarsh.
"That leads us to suspect that our competitor and our partner Mercedes-Benz offered a bit more money."
Whitmarsh said he is also concerned for Hamilton that he may have made his decision hastily, after retiring from the lead in Singapore recently.
"I think it is always bad to make a decision in the aftermath of a bad race," he insisted.
"He was pretty sure that he was going to win that race and it was a disappointment and as I just said it is never good to make a decision in such a situation."
Whitmarsh also said he thinks Hamilton has already regretted his decision "on occasions", but doesn't think the 27-year-old will admit it.
"He is not going to say 'hey, they offered me more money'. He is also not going to say that he's made an awful mistake.
"I hope he thinks today that he's made an awful mistake and I hope he thinks that next year," said Whitmarsh. "He's made that decision and he has to live with that decision."