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Persistently high death rates in Welsh hospitals should be investigated, the medical director of the English NHS has suggested to his Welsh counterpart.

Sir Bruce Keogh said data on rates in six Welsh hospitals was worrying but not adequate to "form a view".

He called for an inquiry in an email sent in November, now released under the Freedom of Information Act.

The Welsh government ruled out an inquiry and said data for England and Wales cannot be compared.

In the e-mail, Sir Bruce told his NHS Wales equivalent Dr Chris Jones: "I cannot vouch for the veracity of these data but they are worrying.

"Given that they represent a potential diagnostic delay and may underpin some of the mortality concerns, it would seem sensible to investigate."

The e-mail comes amid heightened tensions between the governments in London and Cardiff Bay over the performance of the Welsh NHS.

Prime Minister David Cameron has repeatedly criticised the performance of the Welsh NHS and the Labour administration in Cardiff Bay during Prime Minister's Questions.

Statistics published last year by the Welsh government showed 11 out of 17 district general hospitals in Wales had higher death rates than should be expected.

Officials described the findings as a "fire alarm" but they warned that there were major problems with the way the data was collected.

The Welsh NHS has faced problems recently with key targets for A&E missed since 2009, ambulance response time targets hit once since May 2012 and a rise in people waiting more than nine months for their first hospital appointment.

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."

The spokesman said that Sir Bruce had admitted in his e-mail "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 December 10th," said the Welsh government.

"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."

Welsh hospitals with persistently high death rates were brought to the attention of Sir Bruce by Cynon Valley MP Ann Clwyd last year.

She is advising Mr Cameron on how the NHS in England handles complaints.

In the email to Mr Jones, Sir Bruce said the statistics were worrying and offered assistance with an investigation, having recently held such an inquiry into 14 hospitals in England.

He said: "There are six hospitals with a persistently high mortality which warrant investigating. I do not have adequate data to form a view."

But it has been reported that no reply was received from the Welsh government.

The statistics are part of a drive to make the NHS more transparent in the wake of the Stafford Hospital scandal.

Neglect and abuse at Stafford Hospital led to hundreds of unnecessary deaths between 2005 and 2008 and prompted a public inquiry.

Welsh Conservative leader, Andrew RT Davies AM, said: "The concerns raised in this e-mail confirm our serious worries over standards of care in the NHS.

"Professor Keogh's recommendation of 'investigation' into mortality rates should be immediately undertaken and no longer ignored."