More than half of doctors believe that patient safety has deteriorated over the past year, a new report claims.
And nearly three-quarters (74%) of doctors are worried about the ability of their service to deliver safe care in the next 12 months, according to a new report from the Royal College of Physicians (RCP).
A poll of more than 2,100 RCP members from across the UK found that just half of doctors believe their colleagues are confident about speaking up when they have concerns about safety or poor care.
And the majority do not know where to turn even if they felt they could blow the whistle - with 79% saying they do know who their so-called "freedom to speak up" guardian is.
Meanwhile, 37% believe quality of care for patients has deteriorated in the last year and four in five think the medical workforce feels "demoralised".
Doctors said they were "firefighting", "papering over the cracks" and "hanging on by their claws", with one medic claiming: "I feel like I'm on the Titanic."
The authors of the latest report state that hospital services are "under acute strain".
They warned that pressure on the system was having an adverse impact on patients. A number of patients are being discharged before they are medically ready so hospitals can make space, while some patients are not being sent to the appropriate place for care and elective surgeries - including cancer operations - and other clinics are being cancelled so doctors can focus on the most immediate need.
Meanwhile, doctors, nurses and all staff are spread too thinly, risking unsafe or poor care, the authors added.
A separate poll published by the RCP revealed high vacancy rates across the system.
Professor Jane Dacre, RCP president, will speak up about concerns raised in the report at the College's annual conference in Manchester on Thursday.
She will tell delegates: "I am sure these figures will not come as a surprise to anyone in the room.
"The physicians I know, and I include myself, are optimistic, positive, can-do people who produce work-round solutions to intransigent problems. However, they are being pushed to their limits, and no longer are optimistic about the future.
"We worry that there are inherent safety risks in a hospital running at full or over capacity - from an increase in hospital acquired infections to the impact of burnout from overworked staff.
"Doctors and other staff need to know how to raise and escalate safety concerns.
"NHS staff should feel empowered to bring legitimate concerns over patient safety - the evidence shows that where this happens, patient safety incidents decrease."
Commenting on the report, Janet Davies, chief executive and general secretary of the Royal College of Nursing, said: "Hospital doctors are blowing the whistle on sliding standards in patient care - wards are full and without the staff to cope.
"Nursing staff share their fear that things will get even worse in the next year.
"The report is a sharp reminder for the Government of what happens when you neglect the future NHS workforce.
"Whether it's doctors or nurses, jobs are repeatedly advertised but never filled.
"Finance cannot trump patient safety - safe and effective staffing levels are a necessity, not a luxury."
Commenting on the report, Labour's shadow health minister Justin Madders said: "It is an outrage that doctors are being forced to firefight and paper over the cracks because of this Government's neglect and lack of interest.
"The patience of NHS staff is being tested to the limit.
"It is a real concern that doctors have so little faith in the Freedom to Speak Up initiative for whistleblowers.
"The Government needs to do much more to make sure effective measures are in place to support NHS staff who want to raise concerns at work."
Health Minister Philip Dunne said: "We want to make the NHS the safest healthcare system in the world supported by world-class doctors and nurses - that's why there are already 34,800 extra clinical staff, including over 11,600 additional doctors and over 13,400 additional nurses on our wards since May 2010.
"Staff must always feel empowered to raise concerns about safety so we have extended whistleblowing rights to ensure staff have full legal protection when speaking out, making sure every NHS organisation has a Freedom To Speak Up Guardian and strengthening legislation to prevent discrimination against whistleblowers."