The investigation is understood to have been prompted by rising outturn costs on the smart motorways programme and information supplied on Highways England’s special whistleblowing hotline.

The Enquirer understands that the specialist internal counter fraud team has been scrutinising Kier’s M6 contract at junction 13 and 15 around Stoke on Trent for several weeks.

They were investigating allegations concerning procurement and letting of work packages, according to an informed source.

Highways England has also been forensically auditing Kier’s £160m M23 smart motorway project from junction 8 to 10 in Surrey and West Sussex.

This separate investigation was prompted by concerns over how costs spiralled by £60m on the project.

One of the areas under scrutiny by independent auditors is said to relate to steeply rising costs on the earthworks package, which is understood to have soared from around £5m to £48m.

The outcome of the investigations have not emerged yet, but a Kier spokesperson said: “As a trusted supplier to Highways England, Kier fully supports the routine review and monitoring of projects we are delivering on its behalf. There are no ongoing investigations into our work on any smart motorways projects.”

A Highways England spokesperson said: “Highways England operates robust processes for procurement and ongoing monitoring and management of our entire supply chain, to ensure financial probity of taxpayers’ money.

“We regularly and routinely undertake reviews with our supply chain as part of these processes.”