Poor out-of-hours care and a lack of consultants on wards is leading to potentially avoidable deaths of heart patients, a report has warned.
The study, from the British Cardiovascular Society (BCS), found worrying gaps in the care given to patients between 6pm and 8am on weekdays and over the weekend.
It includes the views of consultants and trainee heart doctors who say a lack of staff and poor access to treatment and scans at weekends is contributing to avoidable deaths.
The limited number of consultants working out-of-hours is making the problem worse, they said.
The report identified major issues in delivering care quickly to patients needing pacemakers, those with abnormal heart rhythms and some patients with heart attacks.
Cardiovascular disease accounts for around a third of all UK hospital admissions where people need emergency treatment.
Many of these patients require urgent specialist investigation such as echocardiography - a type of ultrasound scan - or coronary angiography, where dye and special X-rays are used to show the insides of the coronary arteries.
But the report found these services "vary widely" out-of-hours, and said too many patients suffer delays in seeing a consultant when admitted at weekends.
It said: "Patients who require emergency treatment for heart disease should have immediate access to appropriate and timely investigation and treatment at any time of day or night.
"For many inpatients, however, the treatment pathway pauses overnight or at weekends as tests and senior medical staff may not be readily available."
The report said that where consultant cardiologists work as part of an on-call rota, their roles and responsibilities "are often ill-defined".
Furthermore, "in some hospitals providing acute medical care there is only limited or no out-of-hours access to a consultant cardiologist".
Of 1,389 consultant cardiologists working in the UK last year, only just over half (54%) were part of a cardiology on-call rota. Others worked on general on-call rotas but a third of all cardiologists did not work on a rota at all.
The BCS report included the views of 129 junior heart doctors working below consultant level.
Of these, 38% said patients admitted to their units as new cases were not routinely seen by a consultant cardiologist within 24 hours at weekends or bank holidays.
Some 71% said heart patients already in the hospital were also not routinely reviewed at weekends and bank holidays by a consultant cardiologist.
Almost three-quarters (73%) reported a lack of staff available to carry out echocardiography, while almost half (48%) said staff were not available for fitting pacemakers.
However, trainees rated care at their hospital overall as good and said they thought it was easy to get hold of an on-call senior.
A survey of 24 regional specialty advisers - consultant cardiologists who represent the BCS - was also included in the report.
All these consultants "confirmed that there were delays in delivering patient care over the weekend", the report said.
There were also significant concerns about how current staffing levels were impacting on the provision of angioplasty - a procedure used to treat the narrowed coronary arteries in heart attack patients and those with angina.
And while large centres often had appropriate cover, some hospitals provided no consultant cardiologist cover at weekends.
Delays were most severe for patients who needed pacemakers, followed by those who needed angioplasty and then those who needed general cardiology services.
Dr Sarah Clarke, consultant cardiologist and president of the BCS, said: "Patients should be able to access high-quality services whatever time of day or night, or whatever day of the week they present to the healthcare system.
"This report presents challenges to cardiologists, hospitals, ambulance services and commissioners at a time when there are staff shortages and significant financial pressures on the NHS.
"However we have a duty to deliver the best possible care to our patients and it is important that clinicians and hospitals collaborate to address the issues raised by this report."
Professor Sir Nilesh Samani, medical director of the British Heart Foundation, said that "despite marked progress in the delivery of care to patients with a major heart attack, this report shows that for many other heart conditions, care could be improved".
A Department of Health spokesman said: "We are committed to making sure that patients get the same high standard of urgent and emergency care no matter what day of the week it is.
"We want to replace the opt-out clause that means some consultants can currently choose not to work weekends, and are making sure that by 2020 the NHS will have over 11,000 more doctors, including consultants and GPs."