Frank Dobson has warned the Government that allowing Tesco to offer family doctor style services could lead to the closure of GP surgeries.
The former health secretary is leading opposition to the plans to bring private companies in to run NHS services usually provided by family doctors.
Tony Blair defended the idea of allowing supermarkets to open walk-in clinics. "You can get things like routine checks. There is no reason why couldn't have a walk-in centre [at a supermarket]," he said on Channel 4's Richard and Judy Show.
Patricia Hewitt, the Health Secretary, denied that the package of measures aimed at making health care more accessible to patients amounted to a "reform too far". She said: "The NHS has always used the private sector. The majority of GPs have been private, self-employed people contracting their services to the NHS. I don't think there is any issue of principle here."
Ms Hewitt is not expected to face the widespread revolt that Ruth Kelly, the Education Secretary, has encountered with her White Paper on schools. However, some Labour MPs see it as part of Mr Blair's rush to modernise the public services before he steps down.
John McDonnell, the chairman of the left-wing Campaign Group of Labour MPs, said: "There is deep anxiety about the proposals in the health White Paper. It seems the Government is attempting to set up a shadow NHS in the private sector."
Mr Dobson, who ran a group dedicated to fighting private sector involvement in NHS care, said: "It's all very well saying we will get Tescos in but the supermarkets have slaughtered local shops, sometimes to the great inconvenience of the elderly and the disabled. There must be the same danger that supermarket GPs will mean more local surgeries will close in urban areas and in rural areas. When push comes to shove, the first duty of the private sector is to shareholders."
There are likely to be wider concerns about the ability of the NHS to deliver extra services at a time when many trusts are in deficit and cutting services.
Andrew Lansley, the shadow Health Secretary, accused Ms Hewitt of acting like "Patricia in Wonderland" in which all the patients were happy, "doctors wait in supermarkets for patients to drop by and hospitals are empty of patients".He said she was taking "an immense gamble", which could leave hospitals facing more cuts.