The NHS faces the prospect of unprecedented disruption to services from 13 March when junior doctors strike for 72 hours in an increasingly bitter row over pay, morale and safe staffing levels.

As hospitals gear up for only the second such action in the 74-year history of the health service, the Guardian understands several NHS organisations have begun asking students if they can help plug staffing gaps.

The news comes as three health unions agreed on Friday to call off strike action planned for next week, instead entering into pay talks with the health secretary, Steve Barclay, after the government made significant concessions.

The revelation about unqualified students being called on provoked immediate condemnation from doctors’ unions, who raised concerns about safety, and said student medics should refuse to undertake tasks that fell outside their competence or capability.

There are also fears that asking students who have not qualified in medicine to assist with sick patients could see them face legal difficulties if they make mistakes.

Final-year medical students at Brighton and Sussex medical school (BSMS) are among those asked if they would be available “to support clinical activity” in a local NHS hospital during the upcoming strikes.

A spokesperson for East Sussex healthcare NHS trust said: “Utilising student doctors to provide cover during the upcoming junior doctors strike has never been part of our strike planning.

“We are surprised to hear that a student doctor was put in a position where they thought this would be expected of them; this should not have happened, and we will be looking into the issue.”

Dr Paul Donaldson, the general secretary of HCSA, the hospital doctors’ union, said any attempt to deploy students “into such a high-pressure situation” was alarming.

“There is clearly a risk that they may be asked to do things beyond their skill level and without any indemnity to protect them,” he said. “Undoubtedly, proper supervision is going to be difficult with consultants focusing on emergency and acute cases.”

He added: “Dressing this up as a good learning opportunity is plain wrong and reflects a cavalier approach towards patient safety and their duty of care to students.”

Some universities, including King’s College London and Queen Mary University of London, have won praise from doctors’ groups after cancelling student placements due to take place in hospitals on strike days.

The Doctors’ Association UK (DAUK) has called on all universities in England to cancel placements scheduled for strike days over concerns for patient and staff safety.

It said there was a “high” risk of students being asked to “act up” and cover junior doctors’ duties in their absence without the professional security to do so.

“Instructing students to not fill these gaps is unrealistic and does not take into account the power dynamic between students and clinicians,” DAUK wrote in a recent letter to the deans of medical schools.

“Further to this, students lack the professional indemnity to carry out such roles and would be putting themselves at risk doing so. To prevent these issues arising, medical students should not be expected to attend clinical placement during strike action.”

Ray Effah, the co-chair of the British Medical Association (BMA) medical students committee, said medical students should not be asked to fulfil the role of a junior doctor or undertake any tasks “that would sit outside the normal parameters of their placements” to support trusts during strike action.

“It is absolutely crucial that medical students work with relevant supervision and in a way that is safe and within the limits of their competence and capability at all times.

“This includes any periods of pressure experienced by the health service they are learning in, including pressures created by other healthcare workers taking industrial action. The purpose of students on placement is to learn, not service provision.

“If medical students are asked to undertake any tasks that sit outside the agreed learning outcome of their placements in order to manage the fallout of industrial action taking place at their trust, they should refuse to do this and notify the BMA who can advise and support them.”

On Thursday, junior doctors’ leaders met Barclay in London but said afterwards that nothing new was offered and described the talks as “disappointing”. Barclay has repeatedly said he values the work of junior doctors and wants “to continue discussing how we can make the NHS a better place to work for all”.

But the GMB, Unison and the Chartered Society of Physiotherapy unions all decided to take up an offer of fresh talks on Friday after Barclay wrote to them offering to discuss pay – including a one-off cost of living payment for this year.

Unite has opted to remain outside the talks and continue with an ambulance strike next week, citing the preconditions the Department of Health and Social Care placed on the talks.

Meanwhile, a senior NHS leader has warned that the junior doctors’ strike will disrupt services much more than previous walkouts by nurses and ambulance workers – and could imperil patient safety.

“Once the BMA strikes go ahead, they will be greater in scale that we’ve seen before. All NHS organisations or provider organisations have junior doctors, so it will be on a bigger and wider spread [of NHS services],” Chris Hopson, NHS England’s director of strategy, told the Nuffield Trust thinktank’s annual conference.

“The impact will be greater on the NHS. For example, half of all GP practices actually have trainees. It will last longer [than any of the recent NHS strikes], in terms that it will be for 72 hours. And there will be greater numbers of staff involved because just under half of our medical workforce, 61,000 [medics], are junior doctors.”