The government's deliberate dismantling of parts of the NHS risks returning healthcare provision back to the grim and unfair days of the 1930s and 40s, one of Britain's leading doctors has warned.

The sweeping reforms are in danger of turning the service into "an increasingly tattered safety net" for those with complex illnesses such as diabetes and obesity because private healthcare firms will "cherry-pick" patients who are easy to treat, said Dr Mark Porter, the chairman of the British Medical Association's hospital consultants committee.

Its ability to provide a comprehensive and universal service could be lost because of health secretary Andrew Lansley's plan to force hospitals to compete with independent, profit-driven providers for patients, Porter told the Guardian.

Opening up NHS care in England to "any willing provider" could also lead to local hospitals closing down and patients being denied care by private providers because they cost too much to treat, said Porter.

"Very deliberately the government wishes to turn back the clock to the 1930s and 1940s, when there were private, charitable and co-operative providers of healthcare.

"But that system failed to provide comprehensive and universal service for the citizens of this country. That's why health was nationalised. But they're proposing to go back to the days before the NHS," Porter told the Guardian.

Allowing private companies to compete for NHS contracts carries huge risks, Porter added, in remarks that increase pressure on ministers over the health and social care bill going through the Commons.

"It's not that passing the bill will instantly destroy what we have," he added. "But it brings the risk that in some parts of the country, and for some patients, we go back to what we thought we had left behind when we founded the NHS in 1948." He added: "We fear that one unintended but inevitable effect of the bill will be to reintroduce the patchwork provision that marked services in this country before the NHS, where many people did not get the care they needed because while many hospitals gave good service, you didn't get the good service we have today [across the NHS]."

NHS services in some parts of England could be "destabilised" by private firms taking advantage of the controversial introduction of "any willing provider" to win contracts for patients with easy-to-treat conditions. This could lead to some hospitals no longer offering a full range of services and ultimately having to close.

The worst-hit patients would include those with chronic diseases such as obesity, diabetes and heart failure, Porter added. They would have to travel longer distances for treatment.

The government is taking unnecessary risks by imposing market measures on the NHS, as competitive healthcare cannot deliver high quality treatment to everyone.

The NHS could become "a provider of last resort" for patients whose illnesses are of no interest to private firms, added Porterhe said. Once independent providers have signed contracts with the consortiums of GPs they could deny care to patients who would be costly to treat, Porter warned.

The return of unequal healthcare could even see provision starting to resemble that of the US, "where there are quite big geographical disparities in care and tens of millions of people can't get access to high-quality treatment".

Lansley performed his first big U-turn on his plans last week when he agreed to scrap plans to let hospitals undercut the prices they charged for treating patients, which has caused huge alarm in the medical establishment.

The BMA, many other medical organisations and Labour are fiercely opposed to "any willing provider" and hospitals having to compete for NHS work, but Lansley is unlikely to give ground on what he sees as central tenets of the reforms.

The Department of Health responded to Porter's attack by criticising the BMA and insisting that healthcare standards would not suffer. "We are modernising the NHS so we can offer patients high-quality care and improved health outcomes. Doing nothing is not an option", said the health minister Simon Burns.

"We expected some opposition to our modernisation plans from the unions. The BMA have historically opposed giving patients a choice of voluntary, independent and public sector services. But it is not in the interests of patients to bow to their demands.

"We want patients to choose the best care to suit them, but that does not mean a compromise in quality. Only those who meet rigorous quality standards will be able to provide services," he added.