The case was brought by the Office of Rail Regulation after the rail worker contacted a 25,000 volt overhead line near Cricklewood in March 2011.

Rail workers subcontracted by Balfour Beatty were installing high voltage cable next to overhead lines in North West London.

They were working at height on cherry-pickers, moving along the track and installing the cable alongside an existing overhead wire.

One of the high voltage overhead lines which crossed the site had not been isolated.

Those working on the site were unaware of this and the risk to their safety.

As a worker checked the distance between the newly installed cable and overhead wire, he made contact with the live wire and suffered 45% burns which required extensive skin graft surgery.

The sentencing at Harrow Crown Court follows an extensive ORR investigation which found Balfour Beatty had failed to properly plan the work, and had not provided sufficient instructions to workers or appropriate warning about the risk to their lives.

The court heard there was poor communication between the planning and construction teams, which meant Balfour Beatty did not request that Network Rail switch off the electric current from all relevant sections of the overhead wire.

Balfour Beatty had pleaded not guilty to charges brought in connection with the incident, but changed their plea to guilty after the fifth day of ORR’s evidence.

The firm was also ordered to pay £50,000 costs.

Ian Prosser, ORR’s Director of Railway Safety, said: “In this instance, an unacceptable management failure led to a rail worker suffering severe burns after coming into contact with the live overhead wire carrying 25000 volts, which could, and should have been switched off.

“ORR’s investigation revealed that Balfour Beatty’s arrangements fell well short of the standard expected for a construction company operating in a high risk environment.”