NHS trusts have been told by Monitor, the health service regulator, to fill vacancies “only where essential” as it warned that current financial plans are “quite simply unaffordable”.

In a letter to NHS trusts, Monitor’s chief executive David Bennett warned of an “almost unprecedented financial challenge” as he said no stone should be left unturned to find savings.

Bennett wrote in the letter, which was seen by the Health Service Journal, that financial forecasts for 2015-16 are unsustainable as he called for greater savings. The HSJ has reported recently that the provider sector has forecast a deficit of £2bn in 2015-16.

In his letter, Bennett wrote: “As you know, the NHS is facing an almost unprecedented financial challenge this year. Current plans are quite simply unaffordable. As I have said before, if we are to do the best we can for patients we must leave no stone unturned in our collective efforts to make the money we have go as far as possible.

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“We are already reviewing and challenging the plans of the 46 foundation trusts with the biggest deficits. However, it is clear that this process will not close the funding gap and so we need all providers – even those planning for a surplus this year – to look again at their plans to see what more can be done.”

Bennett added: “Ministers have been sighted on these options and are ready to support all providers to reduce their deficits in a managed way although, of course, all actions should be consistent with your responsibilities for safety and the delivery of constitutional standards.”

Andy Burnham, the shadow health secretary, said: “This is a sign of a serious deterioration in NHS finances. It suggests that the financial crisis in the NHS is threatening to spiral out of control and hit standards of patient care.

“The suggestion that hospitals can ignore safe staffing guidance will alarm patients and the government must decide if it will overrule this advice. It will raise further questions about how the government can possibly fulfil commitments on a seven-day NHS without the money to back it up.”