NHS patients are to be offered the option of being treated in a private hospital by 2006.
More than 1m patients will be treated privately by 2008
The government had already promised people needing the most common forms of surgery a choice of five or more hospitals within two years.
But ministers have now told primary care trusts that one of those must be an independent sector provider.
Health professionals said the reliance on the private sector was misguided if treatment could be provided by the NHS.
A spokesman for the NHS Confederation, which represents PCTs, said: "Many primary care trusts are currently offering patients a choice of treatment at more than one private sector hospital.
"However, in some parts of the country that degree of private healthcare provision simply isn't available, so these PCTs may find it difficult to offer a private sector option if required to do so.
Rise in private healthcare
"Some PCTs would expect to offer more than five options to patients, and in many ways it is counterproductive to create choice and then introduce this level of restrictive detail."
And Kevin Curran, the general secretary of the GMB union, accused the government of betraying the NHS.
"For all the lip service we hear paid to public services, private provision in the NHS seems to be flourishing under Labour, helped along by what must now be called the Department of Stealth."
NHS private sector targets
By 2006, patients must be given a choice of five hospitals for the most common treatments.
One of those five should be from the private sector.
By 2008, 15% of NHS operations should be done by the private sector.
The government wants to give patients more choice in where they are treated as part of a drive to improve the NHS.
Private hospitals already perform a number of operations but that is set to rise dramatically over the next few years as the government has promised 15% of hospitals operations - estimated to be about one million - will be carried out privately by 2008 in a bid to drive down waiting lists.
And several PCTs are using private sector nurses to provide care in the community and one London trust has teamed up with a firm to set up a call centre.
Traditionally, private contracts have gone to foreign healthcare firms as the cost of operations by UK firms was more expensive than the NHS price.
But UK companies have now dropped their prices and in April the government signed a contract with two firms to carry out 25,000 operations over the next year in 65 private hospitals across England and Scotland.
PCTs will now be expected to negotiate NHS price operations for the most common types of surgery with private companies as they will be buying in bulk.
This is expected to include cataract and hernia surgery and knee and hip replacements.
The Department of Health letter said: "PCTs will need to show how they are going to ensure effective choice of providers.
"Realistically, this is likely to mean that every PCT should have at least one independent sector provider on its menu of four or five choices for planning hospital care for five of the ten most common elective procedures."
A spokesman for the department said: "This does not reflect a change in policy. It is simply the logical extension of our current policy on choice and plurality."