UK gay rights activist Peter Tatchell's campaign Equal Love is urging Prince William and his fiancee Kate Middleton to support gay marriage.

“Kate and William had a choice,” Tatchell said in a statement. “They could get married, or not.  Same-sex couples don't have this choice. We are banned from marriage by law. 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.”

Currently, gay couples in the UK can enter a civil partnership, but not marriage, and heterosexual couples are banned from seeking a civil partnership.

At a Buckingham Palace rally on Monday, the group will deliver a giant wedding card to the couple.

“We plan to deliver a giant wedding card to William and Kate. As well as wishing the royal couple every happiness, we will highlight the fact that William and Kate can marry, but same-sex couples can't.”

“Even if you don't agree with marriage, the prohibition on gay marriage is homophobic and should be overturned. Don't let the government dictate that LGBTI people can't get married.”

At the group's Facebook event page, more than 200 people have indicated they'll attend the rally.