Public "bickering" between Theresa May and the NHS over funding at a time when the health service is facing severe financial problems is an "insult to taxpayers", an influential parliamentary committee has said.
Few NHS trusts feel they have a plan for meeting financial targets set by the Government, which must now take "targeted action" to avoid a "catastrophic failure" in the service, the Public Accounts Committee (PAC) said.
Amid the strain, the Prime Minister and NHS England boss Simon Stevens are publicly falling out over funding and must find a way to work together "in the best interests of patients", alongside the Department of Health (DH).
The intervention comes after Mr Stevens went into battle with Mrs May over NHS funding, telling the committee in January an extra £10 billion was being made available to NHS England over six years, but overall it had "got less" than set out in its five-year plan.
The comments by Mr Stevens, who is due to appear before the committee again on Monday, contradicted those of the Prime Minister, who has insisted the NHS got all it wanted in the funding settlement.
PAC chairwoman Meg Hillier said: "The NHS as we know it is under threat from growing and unsustainable financial pressures.
"Few trusts feel they have a credible plan for meeting the financial targets they have been set by Government.
"At the same time, the Government seems unable to get its own house in order, plundering NHS investment funds to plug holes elsewhere, and falling out in public over its longer-term strategy.
"Contradictory statements about funding from the Prime Minister and head of NHS England are an insult to taxpayers who deserve an honest, grown-up conversation about future finance and service provision.
"Government's rigid adherence to a set of stock lines about funding, in the face of mounting evidence its plan isn't up to the job, is not it.
"It is inconceivable the Government would allow a catastrophic failure in the NHS and we expect it to take targeted action now to support NHS bodies facing severe financial problems."
An NHS England spokeswoman said that while the PAC is right to highlight the pressures, there is now a "fundamental agreement" on the action needed.
"To that end, the NHS delivery plan being published at the end of March will clearly set out the NHS's realistic and agreed game plan for the next two years," she added.
The committee's report comes after NHS trusts reported a deficit of £886 million in the first nine months of the financial year.
While the sum owed by NHS trusts is down from the record overspend of £2.45 billion in 2015/16, the health service is not on track to meet a year-end deficit target of £580 million.
The committee warned that action being taken to restore financial stability is affecting patients' access to services.
NHS performance is declining against the four hour waiting time target for accident and emergency (A&E), ambulance response time targets, and the target to be treated within 18 weeks of referral, it said.
"Repeated raids" on long-term investment funds to meet day-to-day spending also risk long-term stability, with £950 million taken out of a separate £4.5 billion capital budget in 2016-16 to pay for everyday activities.
The committee called on the DH, NHS England and NHS Improvement to set out a "clear and transparent recovery plan" immediately.
Among other recommendations, the committee called on the Government to publish an assessment of whether the NHS has capacity "to deliver everything they are expected to within the agreed time frames".
By July, ministers should also report back on what they have done to understand the link between financial performance and the impact on patient care.
And they should analyse the impact "extreme" financial pressure in social care is having on the NHS and publish their findings, the committee said.
A Department of Health spokeswoman said: "We are united behind the ambition to make the NHS the safest, highest-quality healthcare system in the world - which also means ensuring financial sustainability for the future, and the hospital sector's financial position has now improved by £1.3 billion compared to this time last year, with 44 fewer trusts in deficit."
Professor Sir Simon Wessely, president of the Royal College of Psychiatrists, said: "This is a critical report which highlights the fact that money allocated by the NHS isn't getting to the front line.
"We know from talking to mental health trust commissioners that there is genuine concern about the future of some services in their area, with some saying that cuts to services will be inevitable.
"NHS leaders have a difficult choice, clearing the deficit or investing in services to help their patients."