Justin Timberlake has offered to pen and perform a one-off song for Ellen DeGeneres and Portia De Rossi's wedding ceremony.

The comedienne-turned-chat show host went public with plans to wed her longtime partner during a taping of her daily show, which aired on Friday.

The big announcement came a day after California's same-sex marriage ban was overturned by Supreme Court judges, who declared the original ruling was unconstitutional.

And Timberlake was among the first to congratulate the couple when he appeared on the show minutes later. Laughing off the suggestion that he should marry his girlfriend, Jessica Biel - and make the event a double wedding, the singer offered, "No... but I'll sing at your wedding."

He insisted he was deadly serious and said he'd put words De Rossi and DeGeneres wrote to music for the special ceremony, which thrilled the host and her girlfriend, who was sitting in the audience.

He said, "You should have an original wedding song; I think it's only fitting. (Write down) everything you want to say to Portia and everything you wanna say to Ellen... and I'll put it to music and it'll be amazing."

(This news article provided by World Entertainment News Network)