Doctors' leaders have expressed concern over "widespread" rota gaps.
The British Medical Association said that staffing and financial pressures facing the NHS were leading to delays in treatment and doctors "juggling" large numbers of patients.
The comments came after a poll of 900 doctors from across the UK found many reported rota gaps while large numbers said they believed that care had "worsened" over the last year.
The BMA poll found that:
- 71% of hospital doctors reported rota gaps in their departments.
- 47% of GPs reported vacancies in their practice.
- Two thirds (67%) said that the delivery of urgent and emergency care services have worsened in the last year.
- 72% believed that mental health care provision had got worse in the last 12 months.
- Seven in 10 (71%) said that GP access had got worse.
Dr Chaand Nagpaul, chairman of council at the British Medical Association, said: "These figures highlight doctors' concerns about a decline in services and widespread staff shortages.
"As doctors, we want to be able to provide the best possible care for patients, but access and quality of care are being affected by staffing and financial pressures.
"The result is delays in patients being treated, and doctors juggling large numbers of patients to compensate for staff shortages.
"This isn't safe for patients and it isn't sustainable for doctors.
"With pressures rising year-on-year, we are calling on politicians to act now.
"We urgently need a long-term solution to the staffing and funding pressures facing the NHS, otherwise it simply won't be able to provide the safe and high-quality care that patients deserve and doctors want to be able to deliver."
A Department of Health and Social Care spokesman said: "To put this survey into context - there are currently a near-record 109,000 doctors working in the NHS and less than 1% of them responded to this 'wide-spread' survey.
"We recently announced a huge 25% increase in training places for doctors, underlining our commitment to ensuring the NHS has the staff it needs both now and in the future."