A High Court battle begins today between the families of asbestos-related cancer victims and employers' insurers who say they are not liable to pay compensation.

The nine-week test case hearing is aimed at settling a fierce legal debate over the point in time when insurers' liability is "triggered" in respect of the fatal lung disease mesothelioma, which takes 30 years or more to develop.

Thousands of families of workers who have already died and others who may contract the disease in the future are awaiting the outcome of the case, which may go all the way to the House of Lords before it is finally decided.

Asbestos was widely used in shipyards, building construction and other industries from the early 1950s until the 1980s before being finally banned in 1999.

The plight of its victims and their families is expected to cost the insurance industry many millions of pounds in the years ahead. The question is, how many millions?

Around 2,000 people a year are diagnosed with mesothelioma in the UK.

The legal issues facing Mr Justice Burton over liability for exposing workers to asbestos dust are complex.

Many employees worked for several organisations during their careers.

Many former employing companies are now defunct.

Some insurers have become insolvent and are dependent on government compensations schemes and reinsurers for funds.

And at what point does the disease begin to "exist" - at the time of exposure to asbestos or when it begins to develop or when symptoms actually appear?

In one of six leading test cases, the family of steel erector Charles O'Farrell, who died in 2003, have won a court judgment for £152,000.

But Excess Insurance, which insured Mr O'Farrell's now-defunct employer Humphreys & Glasgow, is refusing to pay up before the courts have ruled on the legal issues.

In the past, insurers have met the majority of asbestos compensation claims.

But they are now citing a Court of Appeal ruling won two years ago by insurers of occupiers', rather than employers', liability to the effect that the "trigger" is pulled when the disease develops and not at the time of exposure to asbestos.

The O'Farrell test case is backed by trade union Unite, whose joint general secretary Derek Simpson said before today's hearing: "What's at stake here is millions of pounds which should be used to compensate asbestos victims and not pocketed by the insurance industry.

"It is a sickening scenario and we will fight every step of the way to see that insurers are not allowed to pass the buck and dodge their liabilities."

Mr O'Farrell's daughter, Maureen Edwards, said: "It's important to us that we win this test case, not just for our family but for all those families, now and in the future, who will be devastated by this awful disease.

"My dad died a painful death due to mesothelioma and watching him go through it was agonising for all of us.

"But now our grief and sorrow is being dragged out and made worse by the insurers who we feel are doing all they can to get away without accepting any responsibility.

"We will not be able to move on with our lives until we have finally achieved justice for my dad."