LONDON (AP) — The longest planned strike in the history of Britain’s state-funded National Health Service entered its second day of six on Thursday with doctors in England at loggerheads with hospitals over requests for some to leave the picket line to cover urgent needs during one of busiest times of year.

The strike is the ninth organised by doctors in the early stages of their careers in just over a year amid their increasingly bitter pay dispute with the government. Ahead of the strike, plans were laid out for junior doctors, who form the backbone of hospital and clinic care, to return to work if hospitals got overwhelmed.

The British Medical Association, the union that represents the bulk of the 75,000 or so striking doctors, had agreed with NHS managers on a system for so-called derogations, in which junior doctors return to work in the event of safety concerns about emergency care, with hospitals expected to show they have “exhausted” all other sources of staffing before recalling medics.

On Wednesday, the first day of the strike, hospitals made 20 requests for junior doctors to return to work due to patient safety fears, with a number of declaring critical incidents and others warning of significant waits in emergency rooms. None have so far been granted.

In a letter to NHS England Chief Executive Amanda Pritchard, BMA Chairman Professor Philip Banfield said the refusal of hospitals to provide the necessary data “is fundamentally undermining the derogation process.”

In response, the body that represents NHS organisations said form-filling took time and could undermine patient safety.

“Rather than accusing hospital leaders of refusing to provide the required information in full to the BMA, this is more about them needing to limit the precious time they and their teams have available to filling in forms when patient safety could be at risk,” said Matthew Taylor, chief executive of the NHS Confederation.

During the strike, senior doctors, known as consultants, are providing some of the care that their juniors usually provide. But there’s not enough of them to fill the gap and NHS managers have said that tens of thousands of appointments and operations will be postponed because of the walkout.

Britain has endured a year of rolling strikes across the health sector as staff sought pay rises to offset the soaring cost of living.

The BMA says newly qualified doctors earn 15.53 pounds (about $19) an hour — the UK minimum wage is just over 10 pounds (nearly $12.6) an hour — though salaries rise rapidly after the first year.

Nurses, ambulance crews and consultants have reached pay deals with the government, but negotiations with junior doctors broke down late last year. The government says it won’t hold further talks unless doctors call off the strike, while the BMA says it won’t negotiate unless it receives a “credible” pay offer.

The government gave the doctors an 8.8 per cent pay raise last year, but the union says it is not enough as pay has been cut by more than a quarter since 2008.

Junior doctors are due to stay off stay off the job until 7 am on Tuesday.