Equal Love campaigners have said they will deliver a giant card to Buckingham Palace, urging Prince William and Katherine Middleton to support gay marriages.
The campaigners have launched a legal bid to overturn the twin bans on same-sex civil marriages and heterosexual civil partnerships in the UK.
According to The Independent, the wording of the card will read: "Congratulations William and Kate on your Wedding Day. We wish you a happy life together. You can get married, gay people can't. We are banned by law. We ask you to support marriage equality. Equal=Love."
Human rights campaigner Peter Tatchell said denying people the right to marry was "unjust discrimination".
He added: "The royal couple are lucky. They have the option to get married. Gay couples don't have this option. They are barred by law from marriage."
He said: "We urge Kate and William to support marriage equality: the right of same-sex couples to get married. Their support would mean a lot. They take for granted the right to marry. Marriage is something that many lesbian and gay couples want but cannot have."