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 have delivered judgment.

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 the "landmark" ruling, which it said will affect "many of the 2,500 people who are diagnosed with mesothelioma each year".

The Association of British Insurers (ABI) said the judgment provided "clarity and certainty". ABI director of general insurance and health Nick Starling said: "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."

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". MMI chairman Sir John Lovill said: "We welcome the clarity this judgment brings the industry. We will now consider further the detailed implications of the ruling for MMI and communicate to members in more detail our conclusions when that process has been completed."