The medical director of the NHS in England has called for mortality rates and waiting times in Welsh hospitals to be investigated.
Sir Bruce Keogh wrote to Chris Jones, who runs the Welsh NHS, with his concerns.
His e-mail mentions six Welsh hospitals that he describes as having a 'persistently high mortality rate.'
The Welsh Government says the NHS in Wales is "open, transparent and has a higher level of scrutiny than any other part of the UK", adding that comparisons on A&E waiting times and mortality rates cannot be made between England and Wales as both sets of data are calculated differently.
The Welsh Government has issued a strongly-worded statement following concerns over mortality rates in Wales' hospitals.
A Welsh Government spokesperson said: “The NHS in Wales is open, transparent and has a higher level of scrutiny than any other part of the UK.
"Mortality rates in Wales are published on a quarterly basis and latest figures demonstrate clear improvement. If issues are identified, we work quickly to put them right and we do not hesitate to investigate as needed.
“To suggest that the Welsh NHS is covering up high death rates is therefore utterly ridiculous and completely without foundation.
"This allegation appears to be based entirely on an email from the National Medical Director for NHS England, Sir Bruce Keogh, to the Deputy Chief Medical Officer for Wales, Dr Chris Jones.
“In his email, Sir Bruce admits that there is insufficient data to conclude that an investigation should be carried out into any Welsh hospital.
"Moreover, Sir Bruce acknowledges that even where data did exist, he cannot vouch for its accuracy. The points raised by Sir Bruce were discussed with him at a meeting with Dr Chris Jones on December10th.
“The UK Government Statistical Service has also made it clear that comparisons on A & E waiting times and mortality rates cannot be made between England and Wales. Both sets of data are calculated differently."
The Conservatives are calling for an urgent response from the Welsh Government to the concerns about Welsh hospital death rates raised by English NHS medical director Sir Bruce Keogh.
The Welsh Government has responded to the release of Sir Bruce Keogh's concerns about death rates in Welsh Hospitals, saying the medical director of the NHS in England doesn't have the information to support his suspicions.
The man in charge of the NHS in Wales has released his annual report and, despite a year of conflict and controversy across the service, it paints an upbeat picture of progress on many fronts.
David Sissling accepts there were "unprecedented pressures" on NHS Wales last winter.
He also says that acknowledging shortcomings can be "uncomfortable" - but much of the report highlights impovements not problems.
So, is it a system in crisis or a time of rapid improvement?
Andrew RT Davies, leader of the Welsh Conservatives, says investment, "clear leadership and direction" is needed from the Welsh Government for the NHS here.
Speaking to our Health Reporter Rob Osborne, he reiterated his call that an inquiry is needed into standards of care, similar to that carried out by Sir Bruce Keogh into high mortality rates in England.
NHS Wales Chief Executive David Sissling has insisted there does not need to be a major inquiry into the health service here, as the organisation is already doing a lot of work to review and improve.
There have been calls for an inquiry into the standards of care in the NHS in Wales, particular after it was 12 people have died over the last 15 months while waiting for cardiac surgery at the University Hospital of Wales in Cardiff.
Critics have demanded a major inquiry, similar to the one carried out by Sir Bruce Keogh, NHS Medical Director in England, into 14 hospital trusts with unusually high mortality rates there.
Mr Sissling told our Health Reporter Rob Osborne: "We're looking mortality rates and other indicators of quality care, we're bringing to bear external expertise where we need to do so, we're bringing the voice of the patient, the influence of our staff to bear."
"So in a sense we're undertaking this work, and we're doing it across all of the health system, not just looking at a very small percentage of hospitals and organisations. We're doing it for every part of the health service in Wales."
The annual report from the Chief Executive of the NHS in Wales has acknowledged that it has been "a challenging year indeed", with "unprecedented demand" on services during the winter period.
David Sissling has insisted though that there has been improvements to the health service here in "a year of progress".
He said there has been progress on reducing healthcare associated infections, like C.difficile and MRSA, and reducing pressure points in hospitals.
He said the NHS had been more transparent, publishing new data on mortality and infection rates.