Ministers are asking nurses, police officers and other public sector workers to suggest ideas for “fair and responsible” savings as it launches a consultation on cuts.

Most government departments face average spending cuts of 25%, the coalition revealed in its first Budget.

David Cameron is writing to six million public sector workers to ask them for input on how this might be achieved.

Labour say cuts of that size are reckless and will hurt vital services.

Critics have questioned whether cuts of that scale – which the respected Institute for Fiscal Studies think tank say will be the biggest since 1945 – are achievable.

Chancellor George Osborne is already coming under pressure to try and find more savings from the welfare budget, in addition to the £11bn earmarked in his Budget statement, to minimise the impact on budgets for the police and schools.

Ministers will determine the extent of the squeeze faced by individual departments in October’s spending review.

Ahead of this, ministers are urging public sector workers to make clear what services they believe are non-essential and can be discontinued, how services can be better targeted or provided more effectively by private and voluntary groups to save cash.

Search for waste

“We want you to help us find those savings so we can cut public spending in a way which is fair and responsible,” Mr Cameron writes.

“You work on the frontline of public services. You know where things are working well, where the waste is and where we can rethink things so that we get better services for less money.”

Mr Cameron has acknowledged the two year pay freeze announced in the Budget for workers earning more than £21,000 is “tough” and will amount to a pay cut when taking inflation into account.

But he told the BBC that the alternative was thousands of job losses in the public sector.

He has also warned public sector workers they can expect less generous pensions in future as spending on pensions is becoming unaffordable and needs to be reduced as part of efforts to tackle the record Budget deficit

He has suggested that, while entitlements which people have already accrued would be maintained, workers need to contribute more to existing schemes in future and were likely to get less on retirement.

Labour has said the cuts are based on an ideological desire to reduce the scope of the state, rather than sound economic reasons.

“I am really concerned to see a Conservative-Liberal government taking to pieces the welfare state in front of our eyes,” Shadow Education Secretary Ed Balls told BBC’s Newsnight.

“George Osborne does not need to make the cuts he is making, not in this way. It will be damaging to growth and to jobs.”