There is "considerable" risk that the body set to become the economic regulator for health care in England will fail, according to a report.
The study, from the King's Fund, said the large number of objectives proposed for Monitor under the Government's health reforms "may cause confusion and risks diluting the focus of its work".
The Health and Social Care Bill would give the regulator responsibility for setting prices for NHS-funded services, protecting against anti-competitive behaviour and maintaining essential services if providers become financially unsustainable.
The report found there was a "lack of clarity" in how Monitor would work alongside other key health bodies, and a risk that its independence would not be protected from ministerial influence.
It also highlighted the technical challenges of setting prices for such a large number of services - often on the basis of limited information - and hiring sufficient staff with the suitable skills and expertise.
It concluded that the Government should amend the Bill to be more clear about Monitor's objectives, and state a clear process on how the regulator's objectives will be developed without political pressure.
Anna Dixon, lead author of the report, said: "Monitor has been set a formidable task with little precedent and supporting analysis, so the risks of failure are considerable.
"Unless economic regulation is designed and executed well, it may end up imposing more costs than the benefits it delivers.
"As the Health and Social Care Bill proceeds through the House of Lords, we hope that ministers will look again at the lessons to be learned from other regulators and make the changes needed to enable Monitor to succeed in its new role."
A spokesman for the Department of Health said: "The independent NHS future forum has already examined Monitor's duties and powers and has made recommendations we have accepted in full. Monitor has had a strong and successful track record as independent regulator of foundation trusts since 2004 and we are confident it will be able to deliver on its enhanced role."