Patients with unhealthy lifestyles would not be penalised under a proposed NHS constitution, the government says.
Mr Brown wants the NHS to be more responsive to individual needs
Those offered medical guidance would be encouraged to act in a "responsible" way, health minister Ann Keen said.
Newspaper reports had indicated smokers or obese people could be refused treatments, but Mrs Keen insisted: "No-one is suggesting that at all."
A constitution would set out patients' "rights and responsibilities", Prime Minister Gordon Brown said.
The idea is being seen as a major reform, comparable to Mr Brown giving the Bank of England control of interest rates when he was chancellor.
It is being considered as part of Lord Darzi's review of the NHS, ahead of the health service's 60th anniversary.
Mrs Keen told BBC News 24 that most people "actually want to live a healthy lifestyle".
"They also don't want to be lectured at to do that - they want help to do that," she said.
"The NHS is at its best when it is preventing, and helping patients to do that."
She said it was important to ask people "how they feel about their own health" and "what they want from their health".
"I think the responsibility then follows that if clinicians advise that you should lose weight before an operation, or to give up smoking or limit your drinking, that's something I think most people will accept [as] something they want to do."
But Michael Summers, the vice-chair of the Patients Association, said he feared there would still be "conditions" in future for some people.
"We have a National Health Service which says it's free at the point of delivery," he told BBC News.
"It may now come out to be a conditional delivery. That is unsatisfactory and we do not want to go down that route."
Dr Peter Carter, general secretary of the Royal College of Nursing, has said his members would debate the contents of any constitution in the coming months.
"If a constitution reduces the day-to-day influence of politicians in the NHS, that would be a positive step, but ultimately taxpayers need to be able to hold politicians to account," he added.
In a New Year message to NHS staff, Mr Brown said introducing a constitution would help secure the future of the health service for another 60 years.
He thanked employees for their "achievements", saying better care and higher standards over the past decade had reduced waiting times and saved thousands of lives.
But he warned of a major shake-up in the coming year, to tackle hospital infections, improve access to care and create a health service which was "more personal and responsive to individual needs".