More than 60 trainee nursing staff have had their training places cut due to the coronavirus outbreak.
Sheffield Teaching Hospitals trust said the Covid-19 pandemic had "placed intense and unparalleled pressure on the health service".
In a letter seen by the BBC, the 67 affected nursing associates were told they could either leave the trust or take up another role on a lower salary.
Trade union Unison claimed the decision went against national guidelines.
In a letter to the students, Deputy Chief Nurse Karen Jessop said the pandemic had led the trust to "urgently review its current workforce and capacity in order to prepare for and try to meet the forthcoming demand which is likely to be immense".
"This current situation was beyond anything the Trust or you could have imagined or planned for," she wrote.
"Due to the circumstances outlined, the training positions cannot continue as they require time and resources that the Trust no longer has."
A recent report by the Centre for Cities said Sheffield had the highest level of coronavirus infection outside of London.
According to the NHS careers website, "nursing associates work with healthcare support workers and registered nurses to deliver care for patients".
Training for the role is carried out via an apprenticeship scheme and mostly takes place as worked-based learning in hospitals.
Sue Cookman, from Unison, said staff "feel they've been slapped in the face".
"They've been expected within four days to take a pay cut of £1,700 and move on to an area where they don't even don't know where that will be yet," she said.
"If they're going on to areas of the greatest demand it's not rocket science to assume that will be with Covid-positive patients and they're scared."
The BBC has contacted Sheffield Teaching Hospitals NHS Foundation Trust for a response.