Hundreds of thousands of patients have been left without a GP on weekends and evenings due to inadequate out-of-hours services, an investigation has found.
Swathes of the UK had no GP cover on several occasions last year, Freedom of Information data obtained by Pulse magazine shows.
Pulse asked 104 lead out-of-hours commissioners of care how often GP shifts had been left unfilled.
The figures showed that 10 - just over 10% of the total - had occasions when patients had no out-of-hours GP cover.
As a result, patients were told to go to A&E and services had to rely on non-medically qualified urgent health practitioners, or nurses and paramedics.
In Peterborough, there were nine shifts where 230,000 patients were left without access to an out-of-hours GP in 2016. All children under the age of four were "defaulted" to A&E.
In Tower Hamlets, east London, no GP was available on 12 occasions last year. This meant a population of more than 250,000 was told to contact A&E or the "community night team".
Meanwhile, in Doncaster, nurses and paramedics had to cover a population of 300,000 patients on three occasions, with no GP available by phone on one occasion.
In the Scottish Highlands, which has a population of 340,000, there were 31 times when out-of-hours centres were closed due to lack of staff, Pulse reported.
Shifts there had to be covered by centres up to a 30-minute drive away.
In south and west Northern Ireland, the survey found that one GP regularly had to cover 370,000 patients overnight.
Dr Frances O'Hagan, who works for the out-of-hours service locally, told Pulse it was "broken", adding: "Having only one doctor on call used to be rare but is becoming the norm."
Dr Dean Eggitt, who is medical secretary for Doncaster local medical committee (LMC), said "the system is not safe".
He added: "I think 24-hours-a-day patients should be able to contact a senior clinician, a GP.
"If that is not available that is a very serious situation that must be remedied ASAP."
Dr Alan Woodall, an out-of-hours GP in England and a GP partner in Wales, told Pulse: "The pressures on the out-of-hours service are approaching critical.
"I'm the standby doctor for our excellent out-of-hours service, but I cover an area that stretches 800 square miles on the evening shift because there is nobody else.
"It only takes two sick people at either end of the patch to result in doctors having to be pulled in from other patches, leaving their area uncovered. We are constantly trying to firefight demand.
"Because of the rota gaps and pressure on resources, we will have to employ a lot of urgent care practitioners and paramedics to help us cope.
"They are skilled but they're not medically trained, meaning the doctors on duty will also have to provide remote advice."
But Dr Woodall said some people used the service inappropriately, such as a man who rang up and asked for a prescription for Viagra because his girlfriend was coming over.
Other GPs told Pulse that high insurance costs were a factor in why doctors turned down shifts.
South-east London GP Dr Emma Rowley-Conwy said: "We are definitely really struggling to fill our rota; we have seen a huge shift to use of locum agencies, up from 5% to nearly 40%.
"Indemnity is a deterrent as it costs about £10 an hour to get insurance from a defence organisation."
Dr John O'Malley, medical director of Stockport out-of-hours provider Mastercall, told Pulse that many younger doctors wanted a better work-life balance and did not want to do out-of-hours work.
In 2014, the National Audit Office (NAO) reported that, in September and December 2013, 59% of of providers had failed to fill some of their rota hours.