The British Transport Police investigation into Railtrack is believed to centre on the use of labour agencies in its London and South-east station regeneration programme.

Police refused to name companies raided last month as part of the inquiry into alleged fraud and corruption, but it is understood that commercial groups hiring out staff are under investigation.

A spokeswoman for Kier, which is responsible for the South-east regeneration scheme, would only say: “We are helping police with their inquiries, and co-operating with Railtrack on this matter.”

A Railtrack source said it had spoken to the contractors that run sections of the regeneration programme to ask them to tighten up auditing procedures.

Railtrack is itself devoting more staff to auditing contractors’ work and is scrutinising submissions more closely.

But Railtrack does not believe that there are any problems outside the South-east. One company source said: “This is not something that has tentacles spreading throughout the UK.

“With the amount of money we are spending – £2bn a year – and with the way the building industry is set up through networks of subcontracts, there is bound to be some malpractice.”