The independence of a report calling for controversial changes to hospitals has been questioned after emails emerged between its author and Welsh government officials.
The Conservatives say it is "dodgy" and want Health Minister Lesley Griffiths to resign if she knew about the contact.
They say the author was pressured to support changes the minister wanted.
The report's author and the government insist the report was independent.
Ministers have repeatedly stated that the report - entitled Case For Change - is an impartial academic work which cemented and underpinned their argument for major changes to the NHS.
However, questions have been asked since a series of emails emerged in which health economist Professor Marcus Longley asked civil servants for "killer facts" to support the case for changes.
Emails show Prof Longley asking the NHS Wales medical director Dr Chris Jones - also a senior Welsh government civil servant - for further evidence "to sharpen up the document and its impact in supporting the case for change".
It is correspondence such as this that has led the Tories to question the report's integrity.
In another email, Dr Jones asks Prof Longley to make his report "more positive if possible i.e describing a persuasive vision of how things could be better".
Conservative shadow health minister Darren Millar told BBC Radio Wales that health minister Lesley Griffiths should stand down if she knew about the emails.
"We need to understand if the minister knew about how this report was being compiled," he said.
"If she knew about the pressure that her officials were putting on Marcus Longley to produce a report which supported the changes which the minister wanted to make, then she has to go I'm afraid.
"It calls into question her integrity and her ability as a minister."
Prof Longley is a highly respected health economist and director of the Welsh Institute for Health and Social Care at the University of Glamorgan.
In February, the emails show him expressing concern to top Welsh government officials that he does not have enough data to prove services need to be overhauled.
He says: "As we've discussed, a crucial piece in the jigsaw is the argument 'we can't stay as we are: just look at outcomes'. So far, so elusive!"
In another email exchange with Prof Longley in January 2012, another senior civil servant offers to make new NHS performance data available to him, saying there is a "good bit on mortality for the first time e.g weekend effect".
When Professor Longley's report , called The Best Configuration of Hospital Services for Wales was released in May, Health Minister Lesley Griffiths said it was an "independent assessment of the evidence".
She said it "cemented and underpinned" the case for change, adding that Prof Longley had "approached the study from a completely independent position and drawn together the evidence as it is".
She told AMs: "This report constitutes strong evidence from a clinical perspective - not a political one."
Prof Longley's report warns that some services "will collapse" because of staff shortages if changes are not made.
All seven local health boards (LHBs) in Wales are in the final stages of preparing plans for a major reconfiguration of services, which are likely to be highly controversial when published over the coming months.
Prof Longley said: "The Case for Change document is an entirely independent piece of work carried out by myself in partnership with NHS Confederation Wales.
"The Welsh government is by definition the sole custodian of much of the data required in compiling this report, and any correspondence with them was to source this information.
"The report makes a strong and valid case for change in the way our NHS operates and was compiled without bias or influence."
However, opposition politicians criticised the independence of the report.
Mr Millar added: "This evidence stinks. It shows that senior Welsh government officials, the Welsh NHS Confederation and Prof Longley connived together to produce a report with a pre-determined conclusion to pre-empt the unpopular reorganisation of health services across the country.
"Prof Longley's report was far from the independent piece of academic research the minister claimed it to be - it was clearly a 'sexed up' 'dodgy dossier' and should be regarded as such."
Welsh Liberal Democrat leader Kirsty Williams said: "These email exchanges clearly contradict the health minister's statements that this case for change report was an 'independent assessment' and that it was 'impartial, based solely on the evidence'.
"This is an appalling insult to the many people in Wales who are rightfully concerned about the impending NHS reorganisation. How can we now have confidence in the Welsh Labour government's reorganisation process when they sex-up documents to suit their own needs?"
Plaid Cymru health spokeswoman Elin Jones said: "It is very clear that a conclusion was decided on up front and the correspondence illustrates a desperate search for the evidence to substantiate that.
"The report has lost any credibility and is certainly not independent."
Mrs Griffiths's spokesman said: "The Case for Change report produced by Professor Marcus Longley, was commissioned and owned by local health boards and the NHS Confederation.
"Welsh government officials responded to requests for statistical and other information from Professor Longley and the NHS Confederation."
He added: "The Welsh government did not seek to influence or amend the content of the report, as that was entirely a matter for the NHS Confederation and Professor Longley.
"This is another desperate attempt by opposition parties to destabilise a very important debate on the future of our NHS for narrow and cynical political purposes.
"The people of Wales have the choice of believing the evidence based research of an independent and respected academic, or more nonsense from an increasingly desperate opposition. It will be no contest."
In May, First Minister Carwyn Jones told AMs the report was commissioned by the Confederation, adding: "It is not a government report."
Welsh NHS Confederation director Helen Birtwhistle said: "The report by the Welsh Institute for Health and Social Care was commissioned by the NHS in Wales to provide an independent overview of what the clinical evidence says about the best configuration for hospital services in Wales.
"LHBs felt it was critical that the public have access to clear and independent information to help them examine their forthcoming service plans.
"The Welsh NHS Confederation co-ordinated the publication of the final report, and associated communications activity, after its completion in April 2012."