Britain's small army of PC building firms have been warned to keep their act clean or face prosecution under Restriction of Hazardous Substances (RoHS) legislation.

Many smaller companies who assemble computers from imported components assume that it is their suppliers who are responsible for avoiding the use of banned substances, according to the National Weights and Measures Authority, which has won the contract to police RoHS.

"The organisation or person who puts the finished product on the market is responsible," says Chris Smith, RoHS manager for the authority. "Larger companies understand this. But small and medium-sized businesses must realise that they have to think about it."

Substances covered by RoHSS include mercury, Chromium 6, and lead. Manufacturers can ask for exemptions until they find clean alternatives – Intel has just announced that its next-generation 45nm processors will be lead free.

His emphasis at the moment is on encouraging the industry to comply and there have been no prosecutions since RoHS came into effect on 1 July, 2006. But Smith says a number of cases on his files may end up in the courts.

The law is complicated by the fact that Smith's remit also covers components such as graphics cards or processors that are sold as products in their own right. The good news for the estimated 1,000 small PC assembly firms in Britain is that the law allows them to assume that a component from a reputable supplier complies with RoHS.

Smith works by a combination of asking firms for information and making test purchases. He also gets tip-offs. "Sometimes you get a firm complaining that it is being undercut by a company buying cheap components that do not comply with RoHS," he said.

He points out that it is in the interests of manufacturers to comply because it helps them comply with the complementary Waste Electrical and Electronic Equipment (WEEE) legislation, which makes them responsible for the clean disposal of products at the end of their life.

Smith says: "The problem is no that these substances are dangerous when a product is being used. Our concern is with their safety during manufacture and disposal."