Of 730 members voting, 403 (55%) accepted the latest company offer and 327 (45%) rejected it.

The overtime ban began on January 28, with employees seeking a pay rise linked to inflation. The final offer was for 2.5% from July 1, 2010 (with a non-consolidated 2% lump sum or £750 of base salary, whichever is greater); 3% in year two and RPI in year three.

Prospect National Secretary Emily Boase said: “This offer was the best that could be achieved by negotiation and an RPI link was secured in the third year of the agreement through the effectiveness of members’ action.

“Voting was close enough to send a message to the company that the offer was not universally popular. There remain a number of challenges ahead as the company needs to recruit significant numbers of new staff to support its construction programme and the renewal of its transmission network.”

Many of these are specialist roles, which are in short supply, said Boase. “Prospect members work in these shortage occupations, for example as commissioning engineers, project engineers and control engineers. Their skills are at a premium so the company will need to address market pay challenges as a matter of urgency.

“We are proud of the strength of support shown by members during this action and we plan to take forward areas of concern with the company through the National Grid staff forum.”

Prospect has 1,050 members in National Grid, including grid systems engineers, field engineers, gas operations engineers, office-based development engineers and managers working with new connections. Personal contract holders were not balloted.