A senior Liberal Democrat MP has warned Nick Clegg that a compromise deal within the coalition over NHS reforms could be blown apart by a report due this week from a body set up to advise the government on changes to the health bill.

John Pugh, the party's spokesman on health, voiced his concerns over the NHS Future Forum, a group of experts set up to report to the government on what changes should be made. The government is set to accept wholesale the body's recommendations on future competition within the NHS following the "pause" to the controversial bill's progress in parliament.

But Pugh told the Observer he believed there was considerable doubt over whether Sir Stephen Bubb, the expert responsible for the body's conclusions on competition, was sufficiently neutral over the issue. Bubb leads the Adventure Capital Fund, which provides finance for "third-sector" organisations for a return on its investments. Its highest-paid director receives £140,000 a year, according to Companies House, and its clients would stand to benefit from further competition between health care providers.

It is understood that Pugh's concerns over Bubb's views are shared by others who have been heavily involved in the government's listening exercise.

Pugh, co-chairman of the Liberal Democrats' parliamentary committee on health, said: "Despite a growing consensus across the NHS and political spectrum about the appropriate balance to be achieved between collaboration and competition, it could all collapse if an attempt is made to skew the feedback from the listening exercise. In particular, the role of Bubb in chairing the listening exercise on competition is seen by many as a clear conflict of interest.

"Asking Sir Stephen to sum up on competition rules is as neutral as asking Simon Cowell to tell us about the merits of TV talent shows.

"The real problem, though, is Sir Stephen's enthusiasm for better access to NHS work for the charitable sector, which will be a Trojan horse that will allow huge private companies to dismember the NHS in a chaotic fashion."

Bubb has already shown the direction of his recommendations in a blog promising "radical proposals". Two weeks ago he wrote: "I went to a good meeting today with the NHS Confed Partners Network, an umbrella body for all the independent providers in the health service. For political reasons, the private sector were excluded from the Future Forum, so in my area I feel it's only right to ensure I hear their views. And very balanced and sensible they are. I still hold to the view that what matters is what is delivered, not who delivers it."

Last night Bubb, who is chief executive of the Association of Chief Executives of Voluntary Organisations, denied that there was any conflict of interest. He said: "The Adventure Capital Fund, which I chair, is a charity set up to make loans to charities and community organisations. I am immensely proud of the work it has done developing community enterprise in deprived communities."

Pugh's intervention comes as Clegg prepares to persuade colleagues that he has achieved the changes his party demanded at its spring conference. A source close to the deputy prime minister said he would talk through the government's response to the NHS Future Forum's report tomorrow.

"Nick will now be able to present his colleagues with the scorecard and tick off each of their 13 objectives. They'll be there for all to see," the source added. "We're expecting the parliamentary party to be pretty pleased with the result. We have won."

A main concern over the health bill is the potential privatisation of the NHS. However, the Patients Association has warned that the reforms could also wreck the bond of trust between GPs and the sick by making family doctors deny some people treatment for financial reasons. They claim that handing England's GPs control of £60bn of NHS funds could lead to tension between their new financial responsibilities and providing the best care.

The warning comes in the influential group's submission to the government's "listening exercise", seen by the Observer. It says: "No matter what measures of accountability are put in place, patients fear that GPs will face a huge conflict of interest by having to balance their books rather than buying in the best services for patients."

Dr Clare Gerada, chairwoman of the Royal College of GPs, warned that GPs could be tempted to underspend budgets in order to increase their salaries.

The association has put together a strong critique of the government's plans which is based on the views of more than 1,000 patients and people attending six "listening" events it organised.

The submission contains a warning that allowing "any willing provider" to provide NHS services will lead to private firms "cherry-picking the most profitable services. It is feared this policy will result in privatisation of the NHS, depletion of resources from the NHS and instability".

A Department of Health spokesman said: "We need to modernise the NHS now to secure it for future generations. Our plans put power into the hands of doctors and nurses so they can provide the tailored care that patients want, to world-class standards."