Nancy Alperin and Kendra Keller say they were subjected to sexual discrimination and then wrongfully dismissed after reporting health and safety violations at Koko's home in Woodside, in the south San Francisco Bay area.

The lawsuit against the Gorilla Foundation and its president, Francine "Penny" Patterson, Koko's trainer, was filed this week. They want damages of more than $1m.

The foundation has denied the allegations.

Ms Alperin and Ms Keller were hired last year to help look after Koko, who has a vocabulary of more than 1,000 signs. The suit says Ms Patterson pressured them to expose their breasts to the 33-year-old female gorilla several times - sometimes where other employees could see them. They refused.

They were told that if they "did not indulge Koko's nipple fetish, their employment with the Gorilla Foundation would suffer", the lawsuit claims.

Once Ms Patterson allegedly said: "Koko, you see my nipples all the time. You are probably bored with my nipples. You need to see new nipples."

The two also said the Woodside facility had unsanitary and unsafe conditions, including rodents in the food preparation area.

They complained to the Californian health and safety authorities, and were fired a day after inspectors visited the site, they said.