Employees at workshops contaminated by asbestos might not know for years if they face serious health risks as a result.

Slurry containing asbestos fibres leaked into 12 Market Harborough workshops, forcing them to be evacuated.

The decontamination operation cost £500,000.

Debris from asbestos roof tiles was also thrown into the air as workers unlawfully used a high-powered water jet to clean them, Leicester magistrates heard yesterday.

Concept Roofing and Cladding Services Limited was fined £12,000 and ordered to pay £22,375 costs after pleading guilty to breaching asbestos regulations.

Jonathan Salmon, prosecuting for the Health and Safety Executive, said: "It only takes one fibre of asbestos in a lung for a significant potential health hazard to occur.

"We know from other cases that the effects of asbestos take as long as 15 years to become known, perhaps much longer."

He said the company was employed by Leicestershire County Council to clean and repair asbestos roofs at its Court Yard site, in Bath Street.

Tenants complained of contaminated water leaking into their premises after the work began.

Mr Salmon said the contractors should have used a water box cleaning system for most of the roof and a light spray and scrape method on certain parts, to minimise the risk of spray containing asbestos particles getting into the air.

Residue could have been collected in sealed bags.

But he said one tenant saw men using a high-pressure washer, causing water to fly off the roof. Another tenant was soaked by the spray.

Mr Salmon said a county council representative also saw slurry from the roof cleaning overflow a gutter.

He said contamination was discovered in all 12 units. Traces were also found at a bus stop.

On January 22, 2009, the site was evacuated and a three-month decontamination operation carried out.

Stephen Climie, for Concept Roofing and Cladding Services, of Fareham, Hampshire, said it was a subcontractor for Five M.

He said: "The roof was already damaged and leaked.

"There was contamination in units one to four where my clients did not clean the roof."

He said the vast majority of work was done using the approved method.

District Judge David Meredith said: "With asbestos, I have to take into account the risk to the tenants."

He reduced the fine from the maximum £20,000 because of the company's excellent previous record and guilty plea.

After the hearing, HSE inspector Stephen Farthing said: "I am pleased the company pleaded guilty. Every year, 3,000 people are killed by the effects of asbestos."