The women claimed they were being refused the right to perform ground- staff work while pregnant, putting them and their unborn children at risk.
The case was brought on behalf of Anna Gomez de Bonilla, 31, who was suspended without pay in March 1997, when 27 weeks pregnant.
United Airlines, challenging the claim at Croydon employment tribunal, insisted that because the women were employed by an American company based in the United States, they should be subject to US maternity law.
But the Association of Flight Attendants, representing the women, said that because the air hostesses were based at Heathrow, they should be regarded as UK employees for the purpose of legislation.
The women alleged that, under United Airlines' employment policy, they had to work as in-flight cabin crew up to the 27th week of their pregnancy, after which they were suspended on unpaid leave.
The association president, Kevin Creighan, said this placed them and their unborn babies at risk. He stated that there was scientific evidence to show that the health of a foetus was at greater risk when its mother was working in an aircraft.
The women also claimed the right to re-employment in the same job after the 27th week of pregnancy and on their return from maternity leave. Under UK law, women are allowed to seek alternative work in the same company when they are pregnant and in the first few months of motherhood or must be sent home on full pay. No such right exists in US law.
British lawyers believed the case would have implications for other workers employed by American companies in the UK. It might also have affected multinational companies imposing worldwide employment policies in all their countries of business.
Mr Creighan said: "It is a dangerous job, working in a confined space. Statistics show that they get injured. There is therefore a risk to the pregnant women." He also expressed concern over poor air quality in cabins and the risk of injury to the foetus from radiation.
Elaine Aarons, chairwoman of the Employers' Forum on EU Social Policy and head of employment law at Eversheds solicitors in London, said other American companies had tried to make their UK-based employees subject to American employment law. Employees have been made to sign confidentiality clauses and restrictive covenants much stricter than those permitted under UK law, she said. "The test is whether they are working for a UK establishment - if they are, then the [US] law won't stick."
United Airlines, however, argued that because its UK staff fly from Heathrow to destinations all over the world and are paid by an American company, they do not work at a British establishment. The association had brought a similar case for Mrs Gomez de Bonilla in 1996, after a previous pregnancy.
Ms Aarons said before the ruling that if the tribunal found in favour of the company, it could extend the reach of US law in this country. But she added that it should be a matter of fact whether an employee worked for an "establishment in Great Britain" and was therefore entitled to protection under British law.
After the hearing Mr Creighan said: "We are extremely disappointed, because these acts will continue by an arrogant American company on British soil."Reuse content