Mesothelioma most often affects the lining of the lungs
Asbestos-related cancer victims and their families have won an important test case over access to compensation.
Six individual cases were brought against insurance firms which were disputing whether they were liable.
The hearing has hinged on which policy was key - the one at the time of exposure or when a worker becomes ill.
If the High Court had ruled the policy in place at the time of illness was the relevant one it could have made it harder to secure compensation.
Unions described it as a "hugely important victory".
Asbestos-related disease is the biggest cause of work-related deaths in the UK, mainly affecting former workers in shipyards and other heavy industries.
Cancer victims' solicitor Carolann Hepworth reacts to the ruling
The test case concerned a type of cancer called mesothelioma, which can follow exposure to asbestos by 25 to 40 years.
About 2,000 people were diagnosed with the disease last year, but in the future it is expected to cost insurers millions of pounds as all the cases dating back to the 1970s come to light.
Employers take out liability insurance to insure them against the cost of legal action by staff injured at work.
But a group of insurance firms had questioned which policy should be enacted.
They argued during the nine-week hearing over the summer that the policy in place at the time the cancer develops was the one that a compensation claim should be brought against rather than the policy that was in place when the worker was employed by the firm.
They said this was common-sense as victims could have worked for several employers where they were exposed to asbestos.
However, this stance was challenged by families, employers and one insurance firm.
Solicitors representing the families involved in the case said this would make it harder to secure compensation.
One of the key problems is that many modern-day insurance schemes have exemptions for asbestos.
That then only leaves the employer for the victims to go after, but as the disease takes so long to develop many of the firms have ceased to exist.
This was the case for the family of Merseyside gas worker Charles O'Farrell, who died in 2003.
His old employer folded in the 1980s and the insurer, Excess, has claimed the policy from the 1960s should not be valid.
His daughter, Maureen Edwards, said: "My dad would have been proud that we have finally achieved justice for him.
"But he would have been disgusted by the lengths the insurers went to to get out of paying."
Ms Edwards was supported by the union Unite, which was acting on behalf of their members many of whom worked in industries where they were exposed to asbestos.
Unite general secretary Derek Simpson said: "This is a hugely important victory for the victims of the deadly dust and for their families."
He added: "Thousands of men and women across the UK have been negligently exposed to asbestos by their employers but insurers have tried and failed to use legal technicalities to escape their responsibility to pay compensation under the policies they sold to employers.
"They sought to avoid their liabilities while pocketing the money."
Anthony Hughes, president of the Forum of Insurance Lawyers, a lobby group representing some of the defendants, said: "We welcome this clarification of the law.
"We hope this will now unlock the flow of damages to mesothelioma victims."
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