"I'm not a massive believer in marriage," the 52-year-old actor says in the message. "I think my own biography is a testament to that. But I do think that love is the same for everyone. And I do think everyone should have the same rights."
He added, "If you want to get married, whether you're gay, straight, transgender, whatever, you should definitely be allowed to do that."
Joseph Musgrave, the political director of the Out4Marriage campaign said, "Although it's great to have such a well-known celebrity support us, Hugh's key contribution is to emphasize the human element. This is all about the recognition of love and freeing people to decide how they, and not the state, choose to act on that."
Ironically, perhaps, best known for Four Weddings and a Funeral, Grant has had a checkered amorous history. He and actress Elizabeth Hurley were a well-known celebrity couple for 13 years, though during this time he was arrested for picking up prostitute Divine Brown on Sunset Boulevard. He later dated writer and campaigner Jemima Khan.
In November 2011, he became a father as a result of what he called in a statement a "fleeting affair" with Chinese actress Tinglan Hong.
The British government is expected to announce plans to legalize gay marriage in November.
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