An ITV News investigation has revealed the scale of vacancies in NHS hospitals stretching across England, Wales, Northern Ireland and Scotland.
Some hospitals reported more than 1,000 vacancies with the biggest gaps, at every trust, in nursing.
Others admitted crucial roles have remained unfilled for years, despite repeated advertising.
ITV News asked every acute trust in the country to disclose how many staff posts were vacant.
In total 92 trusts responded to our Freedom of Information request.
The new figures underline the scale of the workforce crisis facing the NHS, after years of pay caps for staff in England, Wales and Northern Ireland.
Although hospitals can plug some of the gaps with agency staff, the current workforce crisis means it is growing harder for them to meet demand and ensure patients are safe.
Earlier this year NHS trusts warned of a "breakdown" in workforce planning with fewer trained staff and an increasing number choosing to leave the profession.
Saffron Cordery from NHS Providers, the body in charge of England's hospitals, told ITV News there were a number of implications of the understaffing on the frontline.
She said without adequate staff in place trusts face closing wards or cancelling operations to guarantee safety standards
Ms Cordery, the director of policy and strategy, said bringing in expensive agency staff eats into NHS finances, can hinder morale and disrupt the continuity of patient care.
Health Secretary Jeremy Hunt told ITV News the demand for staff had increased after changes made as a result of the Mid Staffs scandal, which exposed care failings that led to patient deaths.
Mr Hunt said while the number of nurses and doctors in wards had increased the government cannot "magic up doctors and nurses" - though he said it remains an "absolute priority".
He said "big reforms" had been made to nursing training but it will "take time" for new recruits to take up roles.
Yet it's clear supply isn't meeting demand in the NHS and both patients - and over-stretched staff - are paying the price for that.
The key figures
The Belfast Trust, which manages eight of the city's hospitals, has 1,236 unfilled vacancies, with one role first advertised over a year and a half ago.
It is a similar story at some of England's largest hospitals.
But it's not only nursing positions that are going unfilled.
Some hospitals reported worrying gaps in their intensive care units.
The gaps aren't for want of trying.
Derby Teaching Hospitals told ITV News that 44 of its vacant nursing roles and 12 consultant roles were first advertised at least six months ago.
At some hospitals, especially in more rural locations, crucial roles have gone unfilled for years.
Both Northern Devon Healthcare and Yeovil have vacant posts that were first advertised over two years ago, while Great Western Hospitals have a role for chemotherapy sister that has been advertised four times since August 2016.
In Scotland, the Highland NHS Trust reported 357 vacancies with one role that was first advertised over 19 months ago.
Despite no responses from the eight hospital boards in Wales, ITV News has found out there are currently 636 vacancies across the country's hospitals.
Betsi Cadwaladr University local health board is the worst affected with 139 vacancies.
Why are there so many vacancies?
Many chief executives tell me that recruiting - and retaining - the right number of people with the right skills for the job is now their toughest challenge.
The Royal College of Nursing (RCN) believes numbers are being affected by years of pay restraint and increasingly stressful working conditions.
Across the NHS, hospitals are reporting higher than average rates of sickness and burnout.
Kathryn Halford, the chief nurse at Barking, Havering & Redbridge University Hospitals, told ITV News the dependency on nurses from patients is much greater than in the past.
She said one in four patients now has dementia and requires emotional as well as physical support.
Ms Halford added it was not uncommon for some patients to be "very confused", shout or be violent towards nurses.
One in three nurses is due to retire in the next decade and the RCN estimates there are already 24,000 nurse jobs going unfilled.
But in the past year applications for nursing courses have fallen by almost a quarter after bursaries for trainees were scrapped by the government.
And since the vote for Brexit the number of EU nationals registering as nurses in England has dropped by a whopping 92% with the RCN reporting a record number leaving the NHS.
What is the effect on patients?
Many of the hospitals that responded insist they are maintaining safe staffing levels by using agency and bank staff.
But only last month Weston General Hospital had to temporarily shut its A&E department overnight due to consistent inability to recruit enough staff.
At the same time demand for NHS services is increasing.
Over the last six years A&E attendances have risen by 5.9% with little real terms increase in NHS funding.
It is difficult to gauge exactly how much the shortages in permanent staff revealed by the ITV News affect patient care.
But in 2016/17 NHS performance in England deteriorated in a number of important areas.
Around 2.5 million patients spent longer than four hours in A&E - that's 685,000 more than the year before.
NHS England has already admitted it cannot maintain the 18-week referral to treatment standard that ensures patients get prompt care.
What do shortages mean for staff?
The goodwill of staff to cover for shortages is "absolutely not sustainable", an A&E matron has told ITV News.
Alison Leadsham, who works at Barking, Havering & Redbridge University Hospitals, said staff had been going "above and beyond" to keep up a good standard of care but warned low morale would impact on patients.
She said sickness rates had increased while "fatigued" staff were "tired of coming in and working with one less staff member than they should have".