Halliday had appeared in the Mikado

D'Oyly Carte claims she was never officially offered the role and on learning of her pregnancy it became concerned for her health.

Halliday had previously appeared in the D'Oyly Carte version of The Mikado.

The company's general manager Ian Martin said a check had been made as to her availability to appear in the Pirates of Penzance at London's Savoy Theatre.

It was then the company learned she would be at least six months pregnant at the time the show was due to open.

Corsets

Mr Martin said: "She said she was available but that she was pregnant.

"We were casting for the role of teenage virgin daughters of Major-General Stanley and they would be wearing heavily-corseted Victorian costumes which would make it very difficult to disguise her pregnancy.

"We never did make an offer of a part, she was approached purely on availability, but one of the reasons we decided not to offer her the part was the pregnancy."

Mr Martin also said the company had serious concerns about the singer's safety because of the tight-fitting Victorian corsets and awkward scenery.

He added: "The whole idea is that she is a teenage virgin.

"In the play the daughters come across a band of pirates on a rocky foreshore and they scream every time they see a man.

"It would give the lie away to have someone pregnant.

"As I understand it, there is a clause in the law surrounding this which allows authenticity, although it has never been tested. It would have implications across the industry.

Equity has now lodged a case with an industrial tribunal but refuses to comment on the case.