The GMB union claimed to have received information of “a major investigation into allegations of wads of cash changing hands, holidays being paid for and work carried out on individuals’ houses in exchange for work.”

The union also believe that a senior manager from Balfour Beatty and four other senior managers have been sacked or left the job.

A spokesman for National Grid said: “We can confirm that we are working with our contractor Balfour Beatty on their investigation into inappropriate activity around our mains replacement activity in the West Midlands.

“It would be not be appropriate to comment further while the investigation is ongoing.”

Balfour started on the West Midlands deal in April 2013 after securing the work in late 2012 as part of a £1.2bn win alongside the North West Region.

The eight-year Gas Distribution Strategic Partnership (GDSP) contracts involve the design and installation of modern polyethylene gas distribution mains to replace aging metal pipes.

The GMB has now written to National Grid asking for more information.

Gary Smith, GMB National Secretary, said in the letter: “We are receiving information about a major investigation into the GDSP run on your behalf by Balfour Beatty.

“I am told that the investigation underway is into allegations of bullying, mis-reporting, fraud and corruption.

“We understand that gas mains replacement work, running into tens of millions of pounds in value, may have been tainted by allegations of corrupt practice.”