British Transport Police detectives have been probing allegations for more than three years that Metronet managers had close links with subcontractors they awarded tube refurbishment work to.

Police confirmed to the Enquirer that ten people were arrested in March and the the investigation is ongoing.

One detective said: “This is a really complicated case.

“It’s been running for a long time now but new information keeps coming to light and we are working hard to discover the exact circumstances surrounding these contract awards.”

Metronet went into administration in May 2008 and the running of the consortium’s contracts was taken over by Transport for London.

The five shareholders of the firm before it went into administration were Atkins, Balfour Beatty, Bombardier, EDF Energy and Thames Water.