Lord David Owen has accused David Cameron of "reprehensible tactics" by only inviting health bodies that will back his NHS reorganisation to a summit at Downing Street.
The Westcountry peer said the Prime Minister was seeking to "divide and rule" in freezing out the BMA and the Royal College of General Practitioners.
Mr Cameron and Health Secretary Andrew Lansley yesterday held a roundtable discussion with chairs of the emerging GP-led clinical commissioning groups.
A Westcountry doctor invited to the Number 10 talks defended the reform, arguing that giving GPs greater power over budgets was already making the NHS "less frustrating".
But writing in The Guardian newspaper, Lord Owen, a doctor and former home secretary, said: "[Mr Cameron] is clearly trying a policy of divide and rule in the hope that he can break the opposition to the Bill by only asking those he believes will support him.
"The health service is based on much more integrated team work these days right across from nurses, doctors, commissioners and clinicians, and this is not the way to treat an integrated health service. A divided health service cannot function effectively.
"The Prime Minister may think he can outmanoeuvre or override the BMA in the same way that Clement Attlee and Aneurin Bevan did when the 1945 Labour government introduced the National Health Service in the face of opposition of the BMA.
"But the BMA in 1946 was much more evenly divided than they are now and Attlee had a clear electoral mandate from the British people to push through his reforms – something Cameron does not have."
Born in Plymouth, Lord Owen, a former GP, was health minister in the Labour government of the 1970s before forming the Social Democratic Party.
Government ministers denied that key bodies had been deliberately excluded, saying they wanted to hear first-hand from people who are implementing the reforms about how the process is going.
Mr Lansley was confronted by a small group of angry protesters as he arrived at Downing Street for the talks.
As he entered, he said their concerns were based on a "complete misrepresentation" and that none of the reforms involved privatisation of the NHS.
Dr Sam Barrell, a GP who heads the Baywide consortium in Torbay, said after the talks the reforms had already allowed patients to be far more involved in designing services.
"We are getting rid of that duplication of care that sometimes goes on, that breakdown in communications," she said, pointing to a new agreement that allowed her to access blood test records from local hospitals.
"It is much more empowering and less frustrating than it used to be," she said.
Meanwhile, Cornwall MP Andrew George, a member of the Health Select Committee, renewed calls for the Health and Social Care Bill to de dropped, saying it had "no friends".