A number of hospitals have called for striking junior doctors in England to return to work as they face “extreme pressure” during the longest strike in NHS history.

Services across England said they faced “significant demand” just hours after junior doctors embarked on a six-day walkout.

On Wednesday it emerged that more than 20 requests had been made by hospitals for junior doctors to return to work due to patient safety fears.

But the British Medical Association (BMA) said the majority of the requests had been rejected as it criticised NHS officials for not submitting the requests properly and putting the union in an “impossible” situation.

Dr Vin Diwakar, medical director for transformation at NHS England, told the PA news agency that officials had spent weeks doing “intensive preparation” for the strike.

“We have been prioritising emergency care as we have done during previous industrial action,” he said.

On the first day of the strike, more than a dozen hospitals said that emergency services were “busy” and Queen Alexandra Hospital in Portsmouth said its A&E department was “full” as it declared a critical incident.

A critical incident was also declared by NHS Nottingham and Nottinghamshire Integrated Care Board due to “significant pressures on services”.

Other hospitals reported “extreme heightened pressure” and being “exceptionally busy”, with one citing A&E waits of “up to 11 hours”.

But NHS officials stressed that people should still seek care when they need it.

Professor Sir Stephen Powis, national medical director for the NHS in England, said the strike coincides with “one of the busiest and most challenging weeks of the year” as the NHS grapples with increased pressure from winter viruses and a rise in people coming forward who delayed seeking help over the holidays.

Health and Social Care Secretary Victoria Atkins urged junior doctors to call off the strike and “come back to the negotiating table”.

Dr Vivek Trivedi, co-chairman of the BMA’s junior doctors committee, told the PA news agency: “I hope they (the Government) come back to the table now – but from all of the signals they are sending it won’t be until our strike action finishes. And I hope at that point we can come to a resolution.”

The BMA said junior doctors’ pay has been cut by more than a quarter since 2008.

Last summer, the Government gave junior doctors in England an average rise of 8.8%, but medics said the increase was not enough and ramped up strike efforts.

Late last year, the Government and junior doctors entered talks, but after five weeks, the negotiations broke down and more strikes were called.

Junior doctors from the Hospital Consultants and Specialists Association union will join colleagues on picket lines.

Consultants and speciality and associate specialist (SAS) doctors have agreed on a deal with the Government, which is being put to members.