DeGeneres urges gay marriage vote

Same-sex unions were legalised in May, but Proposition 8 seeks to define marriage as between a man and a woman only under the state constitution.

DeGeneres married her partner, actress Portia de Rossi, in August.

'Compassion'

Her TV advert will be screened in the run-up to the state ballot on 4 November.

DeGeneres will tell viewers that opponents of gay marriage are "twisting the truth", and will appeal to people to vote for "compassion and fairness".

Recent opinion polls have shown a swing in the number of Californians who oppose same-sex marriage.