Two former Scientologists have shone a less than flattering spotlight on the controversial organisation, which counts the actors John Travolta and Tom Cruise among its followers, in a landmark lawsuit.

In the test case against the US-based Church of Scientology, Marc Headley and his wife, Claire, have told how they were treated like slaves and forced to work 20-hour days almost continually through the year. Mrs Headley claims she was coerced into having an abortion, while Mr Headley has spoken about how he was subjected to a strange mind-control practise by the actor Tom Cruise.

Both were members of Sea Org, the Scientologists' "religious order" and a supposedly elite vanguard made up of its most dedicated recruits, and signed up to the religion when they were still teenagers.

Members of the order sign a billion-year pledge of loyalty, promise not to have children, and live and work communally.

Mr Headley, 36, says he devoted half his life to Scientology but started to question it while earning 39 cents an hour mass-producing cassettes which, he said, the organisation paid $1 to make but sold for $75. In 15 years, he earned just $29,000, he said.

Now, the couple are seeking back pay and overtime that their lawyer says could amount to $1 million (£660,000) each from the organisation, which was founded in 1953 by the sci-fi writer L Ron Hubbard.

Barry Van Sickle, a California lawyer who represents the Headleys, said: "This is a test case. We didn't do this to make a big bunch of money for the Headleys."

"The idea was that if we could make [the Scientologists] comply with the labour laws, people could get some sleep at night, have some money in their pocket and be harder to control."

The Scientology organisation has denied all the allegations and says the plaintiffs are liars motivated by greed.

But whatever the outcome of the trial, which is scheduled to start in January, the Headleys have already become a thorn in the side of their former church.

Mr Headley, who described in a book how Scientology security guards chased him after he tried to escape the compound.

He also described how, in 1990, he was reassigned so Cruise could practice auditing – the organisation's signature counselling technique – on him.

According to Mr Headley, Cruise spent three weeks practising so-called "upper indoctrination training routines" in which – for hours on end – he would instruct him to speak to a book, bottle and ashtray, even giving the objects orders.

"You tell the ashtray, 'Sit in that chair'. Then you actually go over and put the ashtray on a chair," Mr Headley told the New York newspaper Village Voice.

He said the routine was supposed to "rehabilitate your ability to control things".

Some believe Scientology would suffer heavily without its Sea Org workforce and Mr Headley said the church was terrified that members might try to leave.

He said he lived with 24-hour surveillance, roll call three times a day and censored post.

Sea Org, short for Sea Organisation, was set up by Hubbard to accompany him on his proselytising sea journeys. It members – said by the church to number 6,000 – still sometimes wear pseudo-naval uniforms.

Although they work at all major Scientology centres including Saint Hill, its British HQ Near East Grinstead, many Sea Org members are based at a 500-acre gated compound outside Los Angeles.

Mr Van Sickle said he had an ex-Scientologist client who first signed a Sea Org contract when he was only four years old.

"When you're 15 or 16-years-old, as the Headleys were, they take you out of your home and out of high school, and make you a bunch of promises," said Mr Van Sickle.

He said he was surprised to find that Sea Org members who served In it for 15 years were still on the bottom Scientology rung. "It's a slave labour force that the leaders have no intention of letting rise up the ladder," he said.

Scientology has recently come under attack around the world. It was denounced in the Australian parliament and narrowly escaped being banned in France after being prosecuted for fraud.

It suffered a high-profile defection last October when the Hollywood director Paul Haggis left, claiming it was homophobic and its officials had lied about its practices.

A week ago, a federal judge in Los Angeles dismissed part of Mrs Headley's suit, siding with the Scientologists' contention that she was exempt from wage requirements because she was part of a religious order.

However, Barry Van Sickle, a California lawyer who represents the Headleys, said that the ruling did not address two more serious claims – that the "church" coerces Sea Org members to get abortions and that it engages in forced labour.