Multi-national chemical company Solvay Solutions will have to pay out £440,000 after a dangerous gas cloud wafted across Oldbury from its factory seven years ago.

A supermarket had to be closed, the M5 shut down and thousands of people told to stay indoors during the incident, which started at Trinity Street, on January 2, 2009.

Wolverhampton Crown Court heard how an uncontrolled release of dangerous substances put both employees and members of the public at risk and would have been worse if it had not been a bank holiday.

Workers and residents who came into contact with the gas had breathing problems and streaming eyes as well as other ailments.

During the incident the police set up road closures, part of the M5 was closed by the Highways Agency, and an estimated 4,500 people were asked to stay indoors for two to three hours.

The Health and Safety Executive investigated the incident, inspector Kay Brookes, said: “This was a long and complex case, but at the heart of it lay the fact that this company’s actions caused an incident that affected the public and workers.

“The loss of containment and failure in Solvay’s systems caused huge disruption and the outcomes could have been far worse."

She added: “This case should serve as a warning to other companies dealing with harmful substances that they need to get their processes absolutely right, in order to ensure the safety of the public, if they don’t they will face the consequences.”

The dangerous gas was phosphorus and phosphine which, upon contact with air, spontaneously ignited to produce phosphorous pentoxide.

This reacted with the moisture in the air to produce a mist of phosphoric acid which drifted to a densely populated area.

The HSE investigation found a welded steel bar (‘rodder’) failed at the weld and broke in two. One piece fell back and the other piece pulled clear, leaving an opening through which the dangerous substance escaped.

The incident was reported to the European Commission.

Solvay Solutions UK Limited, formerly Rhodia UK Limited, was fined a total of £333,000 and ordered to pay costs of £110,000 after pleading guilty to an offence under Sections 2(1) and 3(1) of the Health and Safety at Work etc Act 1974.

Solvay had failed to properly assess and act upon the risk of the rodder failing.

Marc Lidderth, area environment manager for the Environment Agency, said: “In our role as environmental regulator and as part of the COMAH Competent Authority, we fully support HSE’s action in bringing this case.

“Environmental and health and safety law set out clear requirements for operators to make sure they take the appropriate steps to run their sites safely, so that people and the environment are properly protected.

“This case is an example of how we take direct action against companies that fail to do this, break the law and create an unacceptable risk to the community.”