Peers have backed the government's position on competition within the NHS in England, despite warnings from Labour that "privatisation" was being introduced at an "alarming pace".
Ministers have promised to "improve the quality and efficiency of health care services" by making procurement decisions "transparent and fair".
Labour had called on peers to block new regulations that "let the privatisation genie fully out of the bottle".
But ministers won a key Lords vote.
Peers voted by 254 to 146, a government majority of 108, in support of the new regulations.
As the upper House debated the changes on Tuesday, Labour health spokesman Lord Hunt of Kings Heath warned: "These regulations are vital.
"The problem is that they hold open the door to a competitive, marketised service in which I am afraid that, despite the wording that has been used tonight, the interests of patients will be not first but last."
They removed the discretion of commissioners to decide when to offer services out to tender, he claimed.
"We face the prospect of NHS services being placed in the middle of a costly bidding war with private companies, discrete services cherry-picked for profit while the NHS is left to run the more complex and expensive services but with less money," the Labour peer added.
"How can that possibly be in the best interests of patients?"
Former cabinet minister Lord Owen predicted that the rules would leave the NHS "massively changed".
He explained: "It will take five, 10, 15 or maybe 20 years, but unless we pull back from his whole attitude there will be no NHS that any of us can recognise, and tonight I feel one feeling only: overwhelming sadness."
But Health Minister Earl Howe said: "There is no government agenda to privatise NHS services - quite the contrary."
He added: "The basic purpose of the regulations is to prevent commissioners failing to think about how the needs of their patients can best be met."
Crossbench peer Lord Walton of Detchant, a former president of organisations including the British Medical Association and the Royal Society of Medicine, told peers: "I am satisfied, after the most earnest and careful consideration, that these regulations do not produce the prospect of privatisation of the NHS."
The coalition government's reforms to the NHS, dubbed the "most radical" in the organisation's history, came into force this month.
More than 200 new organisations were created, including a network of GP-led commissioning groups, to manage the budget, and NHS England, the national board that will oversee the revised system.
In March, Health Minister Norman Lamb announced that the government had decided to amend the National Health Service (Procurement, Patient Choice and Competition) Regulations.
The coalition said these changes would help "ensure that commissioners' decisions on buying clinical services are transparent and fair, and that they improve the quality and efficiency of health care services for patients".
But Labour argues that the regulations "mandate market tendering on the medical profession in all but the most exceptional circumstances".
Shadow health secretary Andy Burnham said: "NHS privatisation is already proceeding at an alarming pace. If passed, these regulations will let the privatisation genie fully out of the bottle."
He had called on peers "to stop this relentless march of privatisation to the heart of the NHS.
"The prime minister needs to be reminded that he does not have permission from the British people to put their NHS up for sale."
On Monday, Labour leader Ed Miliband said "massive increases" in spending on the NHS "won't be available" were his party to win the next general election.