The investigation was launched six months ago following an anonymous letter to Balfour chiefs alleging bribes were being paid by subcontractors to secure work.

Balfour has discovered that payments of up to £1,000 were made to individuals to help secure contracts.

But the bribes were made before Balfour took over the job

Balfour said: “We can confirm that this investigation was sparked by an anonymous letter.

“This letter alleged individuals were receiving payments in return for giving work to subcontractors.  We took this letter extremely seriously and launched an investigation to look into these allegations.

“The letter referred to conduct which allegedly began prior to Balfour Beatty taking over the contract in April 2013, when a number of existing employees were also transferred across to the new contract, through TUPE arrangements.

“After a six month investigation we have found no evidence of systematic bribery and corruption on the contract and nothing on the scale that was alleged.

“We did uncover some evidence to suggest that payments totalling less than £1,000 may have been made before Balfour Beatty took over the contract.

“Processes and controls used by Balfour Beatty and put in place mean it would be extremely difficult for any employee working on the operational side of the business to exert significant, improper influence on the choice of a subcontractor for the contract.

“These measures involve separation of the commercial and operational aspects of letting subcontracts, so the chance of individuals receiving payments for selection of subcontractors on the scale alleged is extremely unlikely.

“There has been able to confirm that there has been no financial loss to National Grid, customers or consumers.

“We did find some evidence to suggest that a small number of employees may have been involved in falsification of dates as to when work was done on “track sheets”.

“The falsification indicated that work was done up to two days in advance.  These track sheets are not used to invoice National Grid, our customer, but are an internal Balfour Beatty recording system used for subcontractor payment and work forecasting purposes.

“This falsification led to no financial benefit to Balfour Beatty or to individuals, nor would it have influenced the choice of a subcontractor.

“The system used to invoice National Grid is entirely separate from the track sheet system and is audited by National Grid.

“As a result of this process, there is no chance of misreporting under the contract either to National Grid or then to the regulator, Ofgem, as a result of this track sheet manipulation.”

Balfour also admitted “there was some evidence to suggest localised bullying.”

The firm added: “Balfour Beatty takes any allegation of this kind extremely seriously and investigates all anonymous tip-offs made either through our ethics helpline which is available to employees and all others.

“We are now considering these findings and next steps.”