The union says the wage increase is effectively a pay cut

The Public and Commercial Services Union (PCS) said up to 20,000 workers will walk out at the Identity and Passports Service and the MoD.

The PCS said employees had become "increasingly angry" over wage rises below the rate of inflation.

The government said increased public service investment meant change, but civil servants were "valued".

The union said hundreds of thousands of civil servants will support Friday's strike by refusing to do unpaid overtime and by taking proper breaks.

A further 20,000 civil servants are to begin a month of industrial action, but not strike, over pay. These will include workers in the Department of Health, Crown Prosecution Service, Land Registry and Learning and Skills Council.

PCS general secretary Mark Serwotka said: "Be it supporting our armed forces across the globe or delivering the passports we take for granted, hard-working staff have become increasingly angry about the way their senior managers have imposed what amounts to a real terms pay cut."

He said tens of thousands of civil servants earn just above the minimum wage and a quarter earn less than �15,400 a year.

'Job cuts'

Mr Serwotka said that dedicated staff had been "battered by massive job cuts and privatisation" and had become increasingly angry about the "government using their pay as an anti-inflationary measure".

"It is high time the government and civil service management started respecting its own workforce and started negotiating with the unions to avoid further industrial action," he added.

The government responded by saying that civil servants were valued highly and there was "absolutely need to strike".

A Cabinet Office spokesman said: "They do a great job for the public."

"At a time when the government is increasing investment in public services, no organisation, including the civil service, can be immune from the need for change, both to ensure value for money and to adapt to new technology.

"There is an established process through which unions can raise any issues of concern they have with these changes - without going on strike."