Health Secretary Jeremy Hunt has been accused of misrepresenting key findings about hospital death rates at weekends by the editor of the British Medical Journal (BMJ).
Dr Fiona Godlee took issue with Mr Hunt over repeatedly claiming there are 11,000 "excess" deaths among hospital patients when setting out his plans for a seven-day NHS.
Mr Hunt is referring to a study co-written by NHS England Medical Director Professor Sir Bruce Keogh, which was published in the BMJ last month.
The study showed that around 11,000 more people die every year within 30 days of admission to hospital on Friday, Saturday, Sunday or Monday compared with other days of the week.
Even when taking into account the fact patients admitted on weekends tend to be sicker, people were still 10% more likely to die if admitted on a Sunday compared with a Wednesday.
Researchers behind the study said a lack of consultant cover and "reduced" services could be contributing to higher death rates.
But they said it would be "rash and misleading" to conclude that an exact number of deaths could have been avoided.
In her letter, Dr Godlee said Mr Hunt's continued use of the 11,000 deaths figure despite this caveat "clearly implies that you believe these excess deaths are avoidable".
She said: "I am writing to register my concern about the way in which you have publicly misrepresented an academic article published in The BMJ.
"Despite the authors' very clear statements to this effect in the paper and elsewhere, you have repeatedly told MPs and the public via media interviews that these deaths are due to poor staffing at weekends, with a particular emphasis on medical staffing.
"This clearly implies that you believe these deaths to be avoidable. I ask you to publicly clarify the statements you have made in relation to this article to show that you fully understand the issues involved."
Mr Hunt is embroiled in a row with junior doctors over a new contract, which he plans to impose from next summer.
The contract will reclassify doctors' normal working week to include Saturdays and up to 10pm every night of the week except Sunday.
Medics argue they will lose out financially as evenings and Saturdays will be paid at the standard rate rather than a higher rate.
They say this amounts to pay cuts of up to 30%.
Mr Hunt has indicated to the British Medical Association (BMA) that he will consider extending the current proposals so that more working hours on a Saturday could be paid at a higher rate.
BMA members are to be balloted on industrial action next month, which could include strike action.