More than 50% of students say their mental health has declined since the Covid pandemic began, says a survey for the National Union of Students (NUS).
Many of the 4,241 students surveyed in November say they have suffered stress, loneliness, anxiety and depression.
Students have seen most face-to-face teaching and social events cancelled.
And the drop-off in interaction with other students appears to hit some students hard, with some finding themselves living completely alone.
Despite these difficulties, only a fifth of the students surveyed had sought mental health support. That rose to 29% for those who reported worsening mental health, the survey found.
Loneliness and isolation appear to have had a huge impact on wellbeing and mood, with many students socialising and meeting others far less after term began.
'I'm not allowed to socialise'
Shaakir, a second-year journalism student at South Bank University, lives alone in a studio flat in London and is one of those who did not seek support.
He has had no face-to-face lessons, and is living in a "bubble of one". He often walks the streets just to get out of his room, he says.
"I don't get to socialise with anyone, as my accommodation - and government - rules are that my bubble is myself.
"I'm not allowed to speak to anyone apart from reception, if I'm collecting a parcel. And I'm not allowed to socialise with any of my neighbours."
Shaakir describes himself as having "zero motivation" - a common symptom of depression.
"On the bad days, usually I'll just stay in bed, stay under the covers, and just sleep as I scroll through my phone, on Twitter or Instagram, whatever it is, and then just go back to sleep."
He adds: "I think at its worst, it's been like three days of just lights out, blinds down. And just the only time I ever leave bed is to go to the bathroom."
But he feels cheated, as he had been expecting in-person teaching once a week.
A South Bank university spokesman said almost all of its courses had face-to-face teaching, and it was made clear at the start of the year if that was not happening.
"If it hasn't happened in this case we apologise," the spokesman added.
He said students had been offered comprehensive mental health support, and those who found themselves living alone had been offered the chance to change accommodation.
But Shaakir feels his university could have done more to reach out to students like him who are struggling, rather than just sending emails, many of which he could not face opening because of his mental state.
The NUS is calling for more investment in student counselling and wellbeing services, as well as individual student unions for the support they offer.
Klaudia, a first-year student at Liverpool Hope University who suffers with anxiety, found herself being forced to isolate within a few days of arriving.
The philosophy student knew no-one, had no face-to-face teaching scheduled - despite that being promised - and was completely alone in her room for days at a time.
She describes how she was not even allowed to move freely in the communal areas of where she was living.
"Every sort of fire door's shut, and everything was cut off .... the living room - where the shared area was - was completely shut off."
Klaudia says she fell into "a very depressing state of mind" in which she felt that everything was "hopeless".
She reached out to university counselling services, which offered her some sessions, but she found it difficult to engage and open up to "strangers".
"It was really hard to discuss the amount of overwhelming things that were happening in my mind at that time," she says.
But she says she survived with the support of her parents, her boyfriend and the kindness of her lecturers..
Eventually she took the tough decision, following advice from her counsellor, to return home to Newcastle to study.
"It was whether I wanted to fall down a hole by myself, or try and fight through it but lose out on things," she says.
Klaudia also says the students stuck inside were monitored by security guards, which she found frightening and led to her feeling like "a criminal in isolation".
"With the rising [Covid] cases and the thinking that there'll be another lockdown, I didn't want to stay there, you know, through that."
A spokesman for Liverpool Hope University said it took the wellbeing of its students extremely seriously, and worked tirelessly to offer mental health and other support.
This includes a team of pastoral advisers who make daily contact with those isolating, mental health and well-being advisers, a laundry service and food deliveries
He added: "Students had access to outside space in a regulated manner according to guidelines.
"Security personnel worked diligently to ensure the safety of both the Hope family and the wider community, in line with government policy."
NUS president Larissa Kennedy said it was no surprise students had experienced deteriorating mental health as a result of the pandemic.
"It is deeply troubling that students are not getting the support that they need, with only 29% of those reporting worse mental health accessing services.
"Students deserve better than their treatment this term. It is time for governments to fund university, college and NHS mental health services to ensure all students can get the support they require.
"Students' unions also need greater investment to continue to provide essential services to students."
A Department for Education (DfE) spokesman said it recognised it had been a very difficult time for students, and protecting their mental health and wellbeing continued to be a top priority.
"We encourage students to contact their university's support and welfare team if they are struggling with their mental health. Many universities have adapted their resources to better support students online and at distance."
The DfE had provided up to £3m to fund the mental health platform Student Space, designed to work alongside university and NHS services, he added.
Support and information on mental health is available on the BBC Action Line page.