National Grid has lost its $40m a year (£26m a year) contract to run the Long Island electricity system in New York state after 13 years, amid an investigation into overcharging for its American services.
Britain's grid operator insisted there was no connection between the inquiry and Long Island Power Authority's decision not to renew its contract when it expires at the end of 2013, 15 years after their relationship began.
"We believe the issues are completely separate," a spokesman for Grid said, adding: "Yes we are disappointed, but it is less than 1 per cent of group profits and we remain committed to our US business."
The Long Island Power Authority did not give a reason for passing to New Jersey's Public Service Enterprise Group the £400m 10-year contract. However, analysts questioned whether there might be a link between the contract loss and the investigation.