The government must spell out how its reforms of the NHS in England can be implemented without hitting patient care, says the NHS Confederation.
Nigel Edwards, the confederation's acting chief executive, says the NHS faces its toughest year ever.
Shadow Health Secretary John Healey said: "Everyone saw big improvements in the NHS from Labour's investment... but now we risk the NHS going backwards."
But Health Minister Simon Burns said: "Reform is a necessity, not an option."
GPs will take control of most of the multi-billion-pound NHS budget by 2013, planning hospital care and services for patients in a sweeping reform that will see primary care trusts and strategic health authorities abolished.
£20bn efficiency savings
Mr Edwards said he supports the objectives of the reforms to the NHS in England - bringing decision-making closer to patients and making up to £20bn efficiency savings.
But he said the government's plans to hand control of NHS budgets and planning to GPs means patients could lose out.
Mr Edwards said: "There is no escaping the conclusion that 2011 will be really tough for the NHS, possibly the toughest year it has faced. If the issues are not fully recognised, they will be dealt with poorly and patients will be the losers."
Mr Edwards said the transition to the new system is causing great anxiety in the NHS and more must be done to win the hearts and minds of the people implementing the changes.
Mr Healey said: "This is a strong wake-up warning to the Tory-led government from the people who run the NHS day-to-day.
"And it comes on top of concern from nurses and GPs that Andrew Lansley's high-risk NHS reorganisation is putting patient care and frontline jobs in jeopardy."
But Mr Burns said: "We are pleased that the NHS Confederation supports the need for reform and for the NHS to make savings.
"These things go hand in hand - the NHS budget is protected, but it must still simplify its structure and cut bureaucracy, which will release further savings to invest in care for patients.
"Reform is a necessity, not an option," he added.
Mid Staffordshire inquiry
Mr Edwards said he also feared a public inquiry into the Mid Staffordshire NHS Foundation Trust, which opened last month, could lead to another unwanted "regulatory revolution".
"That would be difficult for the NHS and all those who want to see the recommendations properly acted upon," said Mr Edwards.
He also urged the government to be "brave" on the question of social care funding.
He said: "The tough questions cannot be ducked any longer. It will be a disaster if this gets put on the back burner once again. This is a moment for the government to be brave and do the right thing for the country."