Whitehall has been told it can offer staff a 4.5% raise - with scope for an extra 0.5% "targeted at lower pay bands" if they deemed it necessary.

Outlining its reasoning in a document published today, the government said: "Departments must ensure pay awards are affordable within their spending settlements, and are aware of the need to balance other budgetary pressures, with consideration of the wider economy and the government's macroeconomic framework."

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Civil service unions criticised the lack of a one-off payment - as offered to those working in health and education - and said the offer was made in the absence of substantial talks.

The Prospect union, which represents "specialist, technical, professional, managerial and scientific staff in the Civil Service" in departments including the Met Office, Health and Safety Executive and Natural England, has announced members will now walk out on 10 May and 7 June.

They will also continue with taking action short of a strike, including overtime bans, which they have been doing since 16 March.

Meanwhile, Mark Serwotka, general secretary of the Public and Commercial Services union, which represents more than 100,000 civil servants, said he was "very confident" members would opt for further strike action.

He said the government's "insulting" offer "will serve only to anger PCS members, stiffen their resolve ahead of the forthcoming re-ballot and increase the likelihood of a new wave of sustained strike action".

"Unlike the health and education unions that have had intensive talks leading to improved pay offers, we were given no opportunity to negotiate - it's the most deplorable way to treat their own staff."

Dave Penman, the general secretary of the FDA union, which also represents employees of the civil service, says he "pleaded" with the government not to go ahead with its recommendation.

"Following months of ministers dragging their feet, the government has decided to shoot itself in the foot over civil service pay," he said.

"Today's pay remit guidance of a 4.5% increase, with no consolidated payment, is unconscionable given the current economic climate that civil servants face."

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Mr Penman said he met with the minister for Cabinet Office this morning and "pleaded with him to pull back from the brink of what will inevitably be a prolonged and damaging dispute".

He said his union's chiefs will meet on Wednesday next week to consider balloting for industrial action in the Civil Service".

The soundings from civil service chiefs will be unwelcome news for the government, which is already grappling with junior doctors' strikes and will be braced for the possibility of further nurses' strikes in the coming weeks.

Last night there were reports that members of the Royal College of Nursing (RCN) were poised to reject the government's pay offer, despite union leaders previously recommending that they accept it.

Teachers are also set to strike later this month and into May after members of the National Education Union (NEU) also turned down the government's pay deal by an overwhelming margin.