Three middle managers have already been suspended during the investigation.

Balfour stressed that “there has been no suggestion or evidence of bribery, corruption or any other impropriety in the award of the contract to Balfour Beatty in April 2013.”

The Enquirer revealed that an investigation has been launched into Balfour’s Gas Distribution Strategic Partnership in the West Midlands to replace gas mains across the West Midlands for National Grid.

Balfour said: “For the past few months Balfour Beatty has been leading an investigation into allegations of improper conduct on our West Midlands gas mains replacement contract, one of two gas mains replacement contracts we have with National Grid.

“Following the award of this contract, Balfour Beatty and National Grid put in place controls to prevent inaccurate reporting of work done under the contract.

“We have found these controls to have been effective and there has been no inaccurate reporting to National Grid or the regulator.

“This investigation is focused on localised behaviour by a small group of individuals working for Balfour Beatty involved with sub-contracting in the West Midlands contract.

“Balfour Beatty has suspended three middle-managers working on the GDSP contract in the West Midlands, while this investigation is on-going.

“We cannot comment further while this investigation continues.”