The Four Weddings and a Funeral star admits he is not a "big believer" in the institution of marriage but is adamant same-sex couples in his native country should be given the same rights as heterosexual unions.
He has joined stars including girl band The Saturdays, singer Paloma Faith, and funnyman David Walliams by backing the Out4Marriage campaign.
In a new video message posted on YouTube.co, bachelor Grant says, "Hello, I'm Hugh Grant. I'm not a massive believer in marriage - I think my own biography is testament to that, but I do think that love is the same for everyone, and I do think that everyone should have the same rights.
"If you want to get married, whether you're gay, straight, transgender, whatever, you should definitely be allowed to do that."
Civil partnerships between same-sex couples are permitted in Britain but the Out4Marriage organisation is calling for gay couples to be given equal rights in law.