Once again Europe has shown itself to be on the side of ordinary working people

John Monks, TUC General Secretary

"It has been a long time coming but it has been well worth the wait. Once again Europe has shown itself to be on the side of ordinary working people," said John Monks, general secretary of the TUC.

Because so many part-time workers are women, employers who exclude part-time staff from occupational pension schemes were found guilty of sex discrimination under European law.

Since a court ruling established this nearly six years ago, almost 60,000 people, mostly women, have made backdated claims.

It's like suddenly changing the speed limit and fining those who have gone above the new limit

Lawyer Tom Flanagan

But companies employing large numbers of part-time staff had argued that under UK law claims could not be backdated for more than two years.

Tom Flanagan of Stephenson Harwood said: "Employers are basically doing no more than UK law permitted them to do, before the ECJ got involved.... it's like suddenly changing the motorway speed limit and fining all those who have gone above the new limit."

Unions welcome decision

Mr Flanagan, who represents businesses up and down the country, says that companies and other organisations employing part-time staff could have to pay between �10bn and �17bn following the court ruling.

But the news was welcomed by unions and workers involved in the case.

Fay Fox, who works for Grimsby College, has belonged to a pension scheme since the mid-1990s, but will now be able to claim back to 1980, when the joined the college.

Her case has been backed by the NASUWT teachers union, and she already has an application pending.

"It seems we have been involved in this for years and years, but the union have continued, despite a lot of setbacks, to support this particular application," she told BBC News Online.

Concern at cost

But Ruth Lea, head of policy at the Institute of Directors, said while businesses agreed part-timers should have the same pension rights as full-time workers they were worried about the cost implications.

She said: "What particularly worries me is that it is so retrospective. We could be talking about businesses having to pay billions of pounds as a result of this decision which is really serious."

Ms Lea said: "How can employers plan when they don't know what their liabilities are? - it really is getting harder and harder to run a business."