Hospital doctors in England on Thursday staged the biggest walkout in the history of the UK's state-funded National Health Service, prompting fears for patient safety.

The unprecedented five-day stoppage over pay and staff retention is the latest in eight months of industrial action across the NHS, which is already reeling from a vast pandemic backlog.

"The NHS has been running on goodwill and now this is the last chance to change that," said 27-year-old junior doctor Arjan Sing, on a picket line outside London's University College Hospital.

He said colleagues were planning to leave for countries that "care about their doctors".

"Doctors have realised they work in a global market, they're not restricted to this country," he added.

Nurses, ambulance staff and other medical workers have all joined picket lines in recent months, adding to pressures on patient appointments.

The industrial action by junior doctors -- those below consultant level -- will run until 7:00 am (0600 GMT) on Tuesday.

It comes against a background of walk-outs across the economy from train drivers to lawyers over the past year as the UK battles a crippling cost-of-living crisis.

Senior hospital doctors, known as consultants, in England will also begin a 48-hour strike on July 20, with radiographers following suit from July 25.

- 'Destructive' -

The bitter row between junior doctors and the government has seen them call for their 2008-9 pay levels to be restored -- something the government says would mean an average pay award of about 35 percent.

The British Medical Association's Junior Doctors Committee says medics have effectively had a 26-percent pay cut in real terms in the last 15 years, as salaries have failed to keep pace with soaring inflation.

The government claims that backdating their pay to reflect inflation since 2008 is too costly and has instead offered an extra five percent, as it battles to reduce inflation.

"Today marks the start of the longest single walkout by doctors in the NHS's history, but this is still not a record that needs to go into the history books," BMA leaders Robert Laurenson and Vivek Trivedi said.

"We can call this strike off today if the UK government will simply follow the example of the government in Scotland and drop their nonsensical precondition of not talking whilst strikes are announced and produce an offer which is credible to the doctors they are speaking with."

Similar stoppages in June and April resulted in massive disruption with hundreds of thousands of hospital appointments and operations rescheduled.

"The complete inflexibility we see from the UK government today is baffling, frustrating, and ultimately destructive for everyone who wants waiting lists to go down and NHS staffing numbers to go up," Laurenson and Trivedi added.

About seven million people were waiting for treatment in April -- a record -- with nearly three million waiting more than 18 months, according to the BMA. (AFP)

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