The so-called weekend effect is "very much a reality" for NHS patients suffering from two common heart problems, an expert has said.
There has been much debate over the phenomenon, which suggests the mortality rate for patients admitted to hospital at weekends is higher than those admitted Monday to Friday.
Two new studies examining patients with two heart conditions in the north of England have lent support to the hypothesis.
But researchers called for further study into the effect on different conditions after the results, published at the British Cardiovascular Society (BCS) Conference, showed varied findings.
The first study examined 42,687 patients with atrial fibrillation, one of the most common forms of abnormal heart rhythm.
NHS patients admitted outside normal working hours were found to have a 10% increased risk of dying in the five years after they were admitted than those admitted during normal hours.
In a separate study, researchers looked at 31,760 heart failure patients discharged from hospitals in the north of England at weekends, and found a 32% increased chance of dying in the following five years compared with those sent home during regular weekday hours.
Dr Rahul Potluri, of Aston University's Medical School, said: "This study shows that the weekend effect is very much a reality for those suffering two of the most prevalent heart conditions in the UK. These patients are, quite simply, more likely to die if admitted or discharged outside regular hours, and that trend is particularly noticeable at the weekend."
Dr Paul Carter, presenting author at the BCS Conference, said: "We took steps during this study to ensure that the comparison between the two groups was as fair as possible.
"It is interesting that both being admitted or discharged from hospital on the weekend confers an increased risk, and one which is sustained in the long term. We cannot comment on the underlying reasons for this from our study but it suggests that the level of support provided at weekends, from all teams involved in healthcare - in the hospital and in the community - should be addressed."
Dr Potluri said: "What this research shows is that the weekend effect is not a universal phenomenon. Across the two conditions we studied, its impact was varied, suggesting it can't be tackled effectively by blanket improvements to care.
"Further research is urgently needed to assess what the implications of the weekend effect are for individual conditions, before any costly changes to services are implemented."
A Department of Health spokeswoman said: "This is further clear evidence of unacceptable variation in care across the week, which this Government is determined to tackle. To create the safer seven-day NHS that we promised the country, we must tackle a variety of factors including weekend staffing levels and access to diagnostics."