Two unions that represent workers at the National Grid are balloting members for industrial action which could affect power supplies in January.

Unions Prospect and Unison will ballot staff at the national power firm this week, in a dispute about a three-year pay deal.

The employer has reportedly offered control centre staff a 2.5 per cent rise for the first year, with 2.25 for the next two years, adding up to a 7.2 per cent rise over three years.

But the unions said staff are angry that this offer is below inflation.

In a letter sent last week, Mike Jeram, head of business at Unison, told Steve Holliday, chief executive of the National Grid: “Our members are disappointed with the company’s attitude towards them. They have worked hard in difficult weather conditions to deliver good profits and don’t feel they are being fairly treated.”

Engineers who maintain the grid system have separate pay deals from the control centre staff but union spokesmen said the field engineers would not be able to do their jobs without this central support.

Industrial action could take the form of working-to-rule and refusing to take overtime, which is crucial in January when the power infrastructure is most in demand and vulnerable to power-outs.

The unions have rejected a three-year offer worth 2.5% from July 1 this year, 2.25% from July 1 2011 and 2.25% from July 1 2012.

“This is less than half the cost of living as, on the 2010 review date, inflation was 5.1%,” said Prospect national officer Emily Boase.

A National Grid spokesman said: “Given the current economic climate, we believe the pay deal offered by the company is a fair deal which gives employees far more security and stability over the next three years than would be offered by a single-year pay deal.”

The union Unite is also expected to add its voice of discontent to the fray this week. However, it may go further and call members to consider strike action.