Relatives of workers who died of an asbestos-related cancer have won a compensation fight at the Supreme Court.
Judges ruled that insurance liability was "triggered" when employees were exposed to asbestos dust - not when symptoms of mesothelioma emerged.
Legal experts say the ruling by the UK's highest court means that employers' insurers will have to pay compensation claims.
Relatives of victims want to make claims on policies from the late 1940s to the late 1990s.
Families started a legal fight for compensation more than five years ago and lawyers say the Supreme Court ruling could affect thousands of claims.
Relatives won the first round of their battle in 2008, when the High Court said firms' insurers at the time workers inhaled fibres were liable.
But two years later the Court of Appeal said in some cases liability was triggered when symptoms developed - sometimes decades after exposure.
Lawyers said the appeal court ruling had left victims' families facing "confusion and uncertainty".
A panel of five Supreme Court justices had heard argument about a group of lead cases at a hearing in London in December and delivered judgment today.
The Supreme Court ruled that the disease could be said to have been "sustained" by an employee in the period when it was caused or initiated, even though it only developed or manifested itself later.
Lord Clarke said: "The negligent exposure of an employee to asbestos during the policy (insurance) period has a sufficient causal link with subsequently arising mesothelioma to trigger the insurer's obligation."
Britain and Ireland's largest trade union, Unite, welcomed today's "landmark" ruling, which it said will affect "many of the 2,500 people who are diagnosed with mesothelioma each year".
It said it had appealed to the Supreme Court after insurance companies were partly successful in the earlier appeal to the Court of Appeal.
Unite's challenge was on behalf of the family of Charles O'Farrell, a retired member who died of mesothelioma in 2003.
Commenting on the Supreme Court's decision, Unite general secretary Len McCluskey said: "This is a landmark ruling which will affect thousands of victims of asbestos.
"It is a disgrace that insurance companies went to such lengths to shirk their responsibilities.
"For callous insurers this means the responsibility holiday is over.
"Unite fought this case to the highest court to get justice for Charles, his family and all victims of asbestos.
"Justice for ordinary people and the ability of trade unions to bring these cases won't be possible if the Government succeeds in slamming the door to justice with their legal aid bill."
Mr O'Farrell's daughter, Maureen Edwards, said: "All I ever prayed for was the right decision. This is the right decision.
"I am delighted for all those families who have been awaiting this result.
"My dad worked all his life and was hoping to enjoy retirement before mesothelioma took him away.
"There was never any question about who was to blame - all this long battle was about was insurers wanting to get out of paying.
"It is very difficult for us to understand the insurance industry's attitude to dying people, an attitude that the Government is going to make worse."
Lawyer Helen Ashton, who represented the lead claimant in the case, said the ruling provided "clarity, consistency and comfort for mesothelioma victims and their families".
The ruling was a "great relief" to her client Ruth Durham, who had continued the legal battle in memory of her father Leslie Screach.
Mr Screach died in 2003 after being exposed to asbestos fibres between 1963 and 1968 while working as a paint sprayer in west London.
His daughter said: "I am delighted to hear of the court's decision which will now see justice done for my father and the other mesothelioma sufferers.
"I was determined to see this through with a positive outcome for all those who, like my dad, suffered so terribly because of someone else's negligence."
The Association of British Insurers (ABI) said the judgment provided "clarity and certainty".
"The ABI and our members are committed to paying compensation as quickly as possible to people with mesothelioma who have been exposed to asbestos in the workplace," said Nick Starling, the ABI's director of general insurance and health.
"We have always opposed the attempt to change the basis on which mesothelioma claims should be paid, as argued by those who brought this litigation.
Municipal Mutual Insurance (MMI), one of four insurance firms which featured in the litigation, said it had "sought resolution" of the issue in order to provide "greater certainty".
"Whilst the ruling does not reflect MMI's favoured outcome, we welcome the clarity this judgment brings as it enables MMI to determine the extent of its liabilities and the available options for the future of MMI and its business," said a spokeswoman.