LONDON—The head of McLaren believes Lewis Hamilton rushed into a decision to leave the team and regrets moving to Formula One rival Mercedes.

Hamilton will end his 14-year association with McLaren at the end of this season after signing a three-year contract with Mercedes, where he will look to add to his world title from 2008.

The Briton made his career choice days after retiring from the Singapore Grand Prix on Sept. 23 with gearbox failure. McLaren team principal Martin Whitmarsh says "it is always bad to make a decision in the aftermath of a bad race."

Whitmarsh says Friday on F1's website that "I think he has on occasions" regretted the move to Mercedes, which has won one race in three seasons.