Kier was contracted by the authority to carry out refurbishment work in Brookside Close as part of Decent Homes, a project aimed at improving council and social housing in the borough. A working for Kier, which is carrying out Harrow Council's Decent Homes programme to bring social housing up to the government standard, fell foul of Health and Safety Executive regulations relating to asbestos, an internal investigation by the company has confirmed.

The low-risk asbestos, in the form of Bakelite cisterns and thermoplastic tiles, was discovered in properties in Brookside Close and Eastcote Lane - but was not sealed inside two plastic bags and thrown into special skips for hazardous waste as it should have been. Asbestos, a once-common fibrous mineral, is the largest cause of workplace-related deaths in Britain and was banned in the 1980s when it was discovered to be carcinogenic.

Kier launched a probe into the matter only after being alerted by Harrow's Labour councillors on Thursday June 24. A summary of the investigation written by Kier's general manager for Decent Homes, Mr C Bunker, said: "Although such non-adherence is non-excusable, containing materials of this nature are only hazardous to health when reduced to small fibrous particles.

"The members of the team whom were in employment with Kier during the time period that these works had taken place, are no longer with the company."

Mr Bunker wrote that, as a result of the "unfortunate incident", more safety checks were being introduced and all relevant staff had been given . Councillor Bob Currie, Labour's spokesperson on housing, and a ward councillor for the area concerned.

Roxbourne, said: "This is simply unacceptable. Over and over again Kier has let us down in implementing the Decent Homes standards. "Dealing with asbestos is a very straightforward procedure which any reputable firm should be able to deal with. "The unions have raised the issue of asbestos with the council over and over again. This is simply the last straw."

Kier was selected in April 2007 to fulfil the five-year ?125m Decent Homes contract to improve council homes borough wide.