David Cameron is to convene a meeting of healthcare professionals on Monday to discuss the implementation of the government's controversial NHS reforms.
The Downing Street gathering is the latest effort to win over critics as opposition to the Health and Social Care Bill mounts.
However, there were claims on Friday night that some organisations most critical of the Bill, such as the Royal College of GPs, had not been invited.
Labour accused Cameron of a "last-ditch desperate bid to shore up collapsing support" for the Bill.
Downing Street would not disclose who had been invited to attend the meeting, saying only that it was a "range of national healthcare organisations and clinical commissioning groups".
A spokeswoman said it was being held "to discuss implementation of the health reforms with representatives from a range of national healthcare organisations and clinical commissioning groups.
"This forms part of the government's ongoing dialogue with health practitioners about the implementation of these reforms."
Shadow health secretary Andy Burnham criticised the prime minister for talking about implementing reforms that were not yet on the statute book.
Labour is calling for the Bill to be abandoned.
"The very title of this hastily-convened event gives the game away," Burnham said.
"It reveals just how rattled the prime minister must be if he is resorting to tactics like this and applying pressure in this way.
"It may sound like a small point to David Cameron but I wish to remind him that he doesn't yet have Parliament's permission to implement reforms nobody wants and for which no one voted.
"This has all the hallmarks of an event thrown together in a last-ditch desperate bid to shore up collapsing support for the Bill.
"It would appear to be so last-minute that a number of important organisations have been left off the invite list, or maybe it's because the PM wouldn't like what they've got to say."
Today, members of the Royal College of Paediatrics and Child Health joined several Royal Medical Colleges, including the Royal College of GPs and the Royal College of Radiologists, in calling for the Bill to be scrapped.
Unions, including the British Medical Association (BMA), the Royal College of Nursing (RCN) and the Royal College of Midwives (RCM) are among those also calling for the Bill to be withdrawn.
Cameron reaffirmed his support for the Bill last weekend after reports that three Tory Cabinet ministers were against the Bill and influential website Conservative Home urged him to drop it.
He insisted he was "at one" with his beleaguered Health Secretary Andrew Lansley.
More than 142,000 people, including footballer Rio Ferdinand and TV star and author Stephen Fry, have signed an e-petition calling for the Bill to be dropped.r Rio Ferdinand and TV star and author Stephen Fry, have signed an e-petition calling for the Bill to be dropped.