The two-day stoppage will be the third in the series of strikes

Members of the Public and Commercial Services Union are protesting over a pay deal imposed last year.

Union leader Mark Serwotka also said a controversial system for judging staff performance was "unfair and divisive".

The government says the strike is neither justified nor necessary and claims it has been timed to hit services offered to vulnerable people.

Bonus system

Job centres and benefit offices across England, Wales and Scotland will be affected by the industrial action.

The government says it has contingency plans in place to keep services running.

The strike is the third of a series of walk-outs - others were held in February and April this year.

It centres around a pay deal for last year which the union says is too low, with starting salaries of �10,300.

The Department of Work and Pensions has yet to start negotiations on the pay for 2004. It waited until this month's government spending deal before finalising its plans but says payments will be backdated.

"The anger over the bonus scheme is palpable Mark Serwotka PCS Union

A bonus system called the Performance Development Scheme has proved a major sticking point.

The union says it is based around quotas so no matter how well people work, half of all staff will receive a lower bonus.

Mr Serwotka said together the two issues had combined to produce a "double whammy" for staff in job centres, benefit offices, the pension service and the Child Support Agency.

"Senior management have seriously misjudged the level of opposition to their tactics of imposing discriminatory and derisory pay deals and their continuing refusal to engage with the union to sort this pay mess out," he said.

'Good award'

"The anger over the bonus scheme that sets staff against each other and has led to some managers refusing to comply is palpable."

He said any disruption to the public was "regrettable" and there should be "serious talks".

The PCS says it now has 97,300 members - and a recent ballot suggested 98% have no confidence in senior managers.

Sir Richard Mottram, permanent secretary at the Department of Work and Pensions (DWP), told BBC Radio 4's Today programme that workers had been given a fair deal.

Its timing is cynical when annual leave is at its peak Government spokeswoman

He said: "I don't think this sort of strike is either justified or necessary.

"In the case of their pay award, we've implemented an award worth more than 5% on average for all our people and 8% for our most junior front line staff. That by any standards is a good award.

"In the case of the dispute over the Performance Development system, we are reviewing that currently.

"We've invited the views of staff and we've asked the unions to contribute as well, so why do we need industrial action?"

'No reason'

A DWP spokeswoman told BBC News Online the contingency plans had been used effectively during the previous strikes.

"We are confident we can maintain services for customers," she said.

The department also insists the bonus scheme is fair, with people's performance assessed relative to their peers rather than against "absolute" standards.

The DWP spokeswoman added: "We cannot see any reason for the PCS union to call a strike.

"Its timing is cynical when annual leave is at its peak and it is clearly targeted at the services we deliver for some of the most vulnerable people in society."