More than a third of GPs on the boards of new NHS commissioning groups in England will have a potential conflict of interest, an investigation suggests.
The British Medical Journal analysed 83% of the 211 boards, which will play a key role in from April, and says potential conflicts will be "rife".
A code of conduct says board members must remove themselves from decisions if they could benefit from the outcome.
The NHS Commissioning Board (NHSCB) says it will issue final guidance soon.
The BMJ says 426, or 36%, of the 1,179 GPs it looked at - who are in executive positions on boards - have a financial interest in a for-profit health provider beyond their own practice.
Their interests range from senior directorships in firms set up to provide services such as out-of-hours GP care, to shareholdings in large private health firms, such as Harmoni and Circle Health.
The information - obtained from Freedom of Information requests and analysis of Clinical Commissioning Groups (CCGs) websites - also shows 12% of the GPs had declared links with not-for-profit organisations that could present a conflict of interest with their commissioning role.
And 9% of GPs declared a conflict of interest through a family member.
The GP-led CCGs will take responsibility, in just under three weeks, for organising NHS care worth about £60bn.
The BMJ's editor in chief, Dr Fiona Godlee, said: "These conflicts will make the commissioning of some services difficult.
"Although board members can excuse themselves from meetings when conflicts arise, this could mean some decisions are made by a group of predominantly lay people."
A spokeswoman for the NHSCB said: "CCGs are under clear duties to ensure that they manage any potential conflicts of interest in ways that preserve the integrity of their decision-making processes.
"This is why it is so vital that everyone working for a CCG or serving on its governing body declares any interests they have."
Six of the eight GPs running Blackpool CCG have declared an interest in the local out-of-hours provider.
The group has chosen to have four lay members - instead of the minimum of two - on its governing body, to help resolve any conflicts of interest.
Dr Laurence Buckman, chair of the BMA's GP committee, said: "While the majority of GPs have no involvement in private companies, we have long called for stronger safeguards against possible conflicts of interest in the new commissioning process.
"In our view, GPs who are directors of, or who have significant financial interests in, companies who might be awarded contracts to provide services should seriously consider their membership of CCG governing bodies.
"Alternatively, they should consider their position within provider companies."
'Blocks' in the system
But some GP leaders have warned that focusing on possible conflicts could distract from the main tasks facing doctors under the new structures.
Dr Michael Dixon, chairman of the NHS Alliance, which represents organisations and individual professionals in primary care, said: "The priority is to move services out of hospital and into primary care.
"The reason this hasn't happened to date is because of blocks in the system.
"It's more important to remove those blocks than be preoccupied with conflicts of interest."
The shadow health secretary, Andy Burnham MP, said: "Patients and public want those entrusted with making decisions about the NHS to have its best interests at heart.
"They will be shocked to learn that so many have a potential conflict of interest."
Dr Clare Gerada, chair of the Royal College of General Practitioners, said: "This is what the college warned about as far back as September 2011 when we issued guidance around the ethics of commissioning.
"Clinically led commissioning on this scale is unchartered territory and GPs are being presented with a tremendous range of new challenges.
"If conflicts of interest in CCGs are not managed effectively, the consequences could badly undermine the confidence of regulators, providers and, most importantly, patients, in the system."