Universities in the UK are being urged to scrap plans for face-to-face teaching until Christmas in order to prevent a second wave of coronavirus.
Academics’ union, UCU, said more than a million students moving around the country was “a recipe for disaster”.
UCU leader Jo Grady said universities were not prepared and risked becoming the “care homes of the second wave”.
But university bosses say they have worked hard to plan for a safe return to all aspects of student life.
When students return in September, they will be expected to abide by new policies on the use of face coverings and social distancing protocols.
Meanwhile there will be safety partitions to keep them apart, as well as more cleaning and ventilation and smaller class sizes where face-to-face teaching takes place.
When national lockdown measures were brought in at the end of March, universities cancelled lectures, tutorials and some exams.
Many continued their lessons and lectures online however, and even held final exams remotely.
Over the summer, universities have been preparing for the return of their students with most promising a mix of face-to-face and online learning – or “blended learning” – with students taught in extended “bubble” groups.
Larissa Kennedy, the president of the National Union of Students, told BBC Breakfast universities needed to “refrain from in-person teaching” unless it was absolutely necessary, for example, sessions in technical labs or studios.
“We need to ensure remote learning is as accessible as possible and I think we need to do that really quickly,” she added.
Meanwhile few, if any, universities have taken the step of telling their students not to move into their accommodation.
Ms Grady said having tens of thousands of students heading into cities across the UK “risks doing untold damage to people’s health and exacerbating the worst public health crisis of our lifetimes”.
And she told Breakfast the mass migration of a million students could prompt a “silent avalanche of infections”.
“There is far more of a risk than perhaps the general public has appreciated,” she said.
She said she was particularly worried about the risk of the virus spreading in cities with big student populations and in areas with fewer cases at present.
Ms Grady told the BBC: “If we look at some key cities: Manchester has 100,000 students; Birmingham isn’t far behind with 80,000; and Leicester has 40,000.
“These are all cities that have had some form of local lockdown and have come out of it or are about to go into one.”
“We are really worried about young people congregating in halls of residence and just what this will mean in terms of transmission,” she added.
The already difficult situation, with universities doing their best to bring in socially distanced ways of working, has been made worse by the higher than expected number of students this year, she said.
Prof Carl Heneghan, director of the Centre for Evidence-Based Medicine at the University of Oxford, told Breakfast “right now it is as safe as it ever has been to go back” to university.
He suggested universities put back face-to-face teaching until the end of the academic year in spring or summer 2021, when the threat of the coronavirus may be reduced.
“Can we do something over the summer, can we put a summer school on?” he asked, adding: “We need to maximise that experience.”
‘No regular testing’
The exams crisis over the summer meant many more students ultimately achieved the grades needed to get into university, after the government’s last-minute change of mind on school-assessed grades.
Universities UK president Julia Buckingham said: “Many staff want to return to in-person teaching, research and other activities where it is safe and appropriate to do so.”
They are also mindful of the benefits of in-person teaching and support for students’ well-being and development, she said.
“The dedication shown throughout the lockdown and into recovery has been remarkable,” she added.
A Department for Education spokesman said: “We are confident that universities are well prepared for the return of students by taking measures such as introducing social distancing on campus, limiting travel requirement for classes and staggering teaching across extended days to reduce numbers on site.
“Opening universities is a part of the Prime Minister’s cautious roadmap, and it is important that we continue to open education settings wherever it is safe to do so.
“We support face to face teaching only where possible and if safety guidelines are followed, but know that high quality online teaching can also be delivered if necessary.
“We are keeping our guidance under constant review, and are currently updating our advice on reopening higher education buildings and campuses to reflect the latest public health advice, including on face coverings, local lockdowns and test and trace.”