Dozens of hospitals are facing acute pressure and social care services are being scaled back because of NHS deficits, two separate reports say.
The NHS deficit tops £500m
The Liberal Democrats said 16 NHS trusts, running 28 hospitals, were facing "high pressure".
They said services would be scaled back and hospitals might even close.
And a Local Government Association survey of 55 councils in the areas affected by NHS deficits said some services had been withdrawn.
The health service finished last year more than £500m in deficit, with one in three NHS bodies failing to balance their books.
The problems have already led to jobs being cut, operations delayed and wards closed.
But the Lib Dems are warning more drastic measures may have to be taken at the 16 worst hit trusts.
They analysed how deficits combined with government reforms introducing more competition in the hospital sector could affect 152 NHS trusts in England in the coming years.
The Lib Dems said nine of the 16 trusts were in London and the south east with nearly a quarter of the hospital network in the capital facing "strong pressure".
The West Hertfordshire Hospitals NHS Trust, which includes St Albans City, Hemel Hempstead and Watford hospitals, is facing the most problems, the research said.
The trust has warned that, if cuts are not made, a £100m deficit could be run up by 2010.
Lib Dem health spokesman Steve Webb said: "Despite ministerial denials, it will be patients who suffer from cuts in frontline services."
The Local Government Association (LGA) survey, compiled in conjunction with the NHS confederation, also revealed the pressure from NHS deficits was hitting social services.
Some 55 of the 78 local authorities in areas with deficits replied to the poll.
Seven in 10 councils said they had suffered because of the financial problems, reporting funding for joint NHS and local government projects had been withdrawn and that there was a "sharp increase" in the referral of patients who would normally have been cared for by the NHS.
The councils reported this had led them to withdraw services from people with low-level care needs and increase waiting times for social care assessments.
Councillor David Rogers, the LGA's social care spokesman, said: "Health and social care are two sides of the same coin. It is impossible not to cut services on one side without hurting the other."
Shadow health minister Stephen O'Brien said: "Despite the government's repeated, barefaced denials of cuts to frontline patient services we see here stealth cuts to social services caused by the NHS deficits."
But the government said local authorities were receiving extra funding to boost social services.
And Health Minister Lord Warner said the Lib Dems were behind the times as the overspends were being tackled in the worst areas with teams of financial experts being sent in to give advice.
"This helps trusts cut their costs and become more efficient - for example, doing more operations as day surgery and reducing the length of hospital stay."