NHS bosses fear patient safety could be compromised during this week’s junior doctors strikes if medics do not honour an agreement to abandon picket lines if hospitals become overwhelmed during the winter crisis.

Hospital bosses can ask the British Medical Association (BMA) to allow junior doctors to return to work to help if an emergency arises during their six-day strike starting on Wednesday.

But there is concern among health trust leaders that the doctors’ union could reject such “recall requests” – or take worryingly long to consider them – despite “highly vulnerable” hospitals having too few staff on duty to cope with a surge in patient numbers.

A spike in cases of flu, Covid and norovirus has left the NHS under intensifying strain in the first week of the new year, a period in which its winter crisis often bites.

On the eve of the 144-hour strike – the longest in NHS history – the NHS Confederation, which represents trusts, urged the BMA to ensure the “recall system” worked reliably if it was triggered.

“With the next round of junior doctors strikes coinciding with what is always an exceptionally busy week for the NHS, health leaders hope that escalation plans run smoothly and with a shared understanding that protecting patient safety is the most important priority,” Danny Mortimer, the confederation’s deputy chief executive, said.

Under an agreement between the BMA and NHS England, hospital bosses can ask the doctors’ union to allow striking medics to cross picket lines to work if “unpredictable events, major incidents and unexpected and extreme circumstances” occur.

However, it is unclear if the BMA will agree that any situation in which a hospital ends up struggling over the next week because of winter pressures counts as a “major incident”.

A senior NHS official said problems could arise during the strike because of potential disagreements over the interpretation of how serious an incident had to be before a recall request would be approved. There is also a risk that the BMA may take longer than NHS bosses feel is necessary to consider a trust’s request, assess the evidence presented and make a decision.

The BMA and NHS England reached their concordat on recall requests on 19 December after the union rejected calls to grant England-wide “derogations” – or exemptions – for junior doctors to not strike in three NHS services which offer life-or-death care, in which patients’ health could be harmed if they do not need to receive urgent treatment. They were for fast-progressing cancers, corneal transplant eye surgeries and births by time-critical inductions and caesarean section.

“There was a clear preference for agreed derogations for key services. But in their absence if local organisations [NHS trusts] feel the strikes are directly and significantly compromising patient safety, they need assurances this will be understood and acted upon swiftly by the BMA in allowing the striking doctors to temporarily return to work,” Mortimer added.

The BMA has stressed that, like NHS England, it is fully committed to patient safety. But it has become more sceptical in its attitude to recall requests after the Weston general hospital in Somerset last April asked for seven A&E doctors to return to work as an emergency even though, according to the BMA, it later emerged that they were not needed. The union accused the trust of having “deliberately misled us”.

The risk of hospitals and the BMA ending up at loggerheads over such requests appeared to increase when the union signalled that it would not approve recall requests from hospitals laid low by winter pressures.

“We have given trusts enough notice and expect hospitals to have planned staffing for urgent, emergency and critical care services during strike action,” a BMA spokesperson said.

“There is a well-established process for requesting derogations, which includes the recall of staff in a mass casualty event, and we take account for alternative sources of staffing. But derogations should be a last resort. We will continue to work with trusts to ensure the process of derogation requests is fair and properly followed, whilst also protecting doctors’ ability to strike.”