A two-month "listening exercise" in which medical professionals will be asked to contribute to a review of changes to the NHS has been thrown into doubt by a confidential memo highlighting a series of government red lines that must be maintained.

As David Cameron and Nick Clegg joined the health secretary, Andrew Lansley, on Wednesday to launch the exercise at a hospital in Surrey, the memo by NHS chief executive David Nicholson indicated there may be little room for manoeuvre in reworking the health and social care bill.

The memo drew a red line beneath the fundamental planks of the bill that are not for changing: GP consortiums, an independent commissioning board to oversee them, every hospital to become a foundation trust, and Healthwatch and primary care trusts to be abolished by 2013.

The memo said there would be delays in setting up Monitor, a regulatory body for bringing competition in the NHS, to which many object, which will slip to July 2012, and the abolition of strategic health authorities will also be delayed to the same date.

The memo is likely to be seized on by Labour which says that the "listening exercise" is more of a PR exercise.

Cameron, speaking at Frimley Park hospital, said the two-month "pause" in the bill offered health workers an opportunity to amend the government's plans that will see 60% of the NHS budget handed to GP-led consortiums. Professor Steve Field, former chairman of the Royal College of General Practitioners, will chair a hastily assembled forum of medical experts that will report by the end of May or the beginning of June.

The prime minister said: "Let me be clear: this is a genuine chance to make a difference. Where there are good suggestions to improve the legislation, to improve the changes, those changes will be made." But "the status quo is not an option", he said; the NHS could deliver better care, saving more lives, if doctors rather than "managers" were put in charge.

Lansley appeared to indicate on Monday, when he announced the "pause", that the changes would not be far-reaching when he said the bill would be amended "in the normal way" when it is revived in mid-June. But Clegg made clear on Tuesday that "substantive changes" would have to be made to the legislation.

Change to the NHS was vital because of the increasing numbers of elderly people who will need treatment, Cameron said. "If we want to keep an NHS that is free at the point of use we have got to make the NHS more effective," he said.

He claimed the changes were already working, with 3,000 fewer managers and 2,500 more doctors in place, and many more people accessing life-saving drugs through the cancer drugs fund. "But we recognise there are questions," he said.

At Frimley Park, breast cancer surgeon Ian Laidlaw said he was concerned that giving groups of GPs more say in what hospitals do could threaten, not enhance, the latter's success. The multidisciplinary teams they had built up in cancer care were delivering exceptional outcomes, he told the politicians. "These have been hard won, and I'm really concerned that the commissioning driven by GPs who also hold the budget will look for cost-efficient choices which will fragment the very teams that have delivered the service."