Psychiatrists are warning of a "tsunami" of mental illness from problems stored up during lockdown.
They are particularly concerned that children and older adults are not getting the support they need because of school closures, self-isolation and fear of hospitals.
In a survey, psychiatrists reported rises in emergency cases and a drop in routine appointments.
They emphasised that mental-health services were still open for business.
'Patients have evaporated'
"We are already seeing the devastating impact of Covid-19 on mental health, with more people in crisis," said Prof Wendy Burn, president of the Royal College of Psychiatrists.
"But we are just as worried about the people who need help now but aren't getting it. Our fear is that the lockdown is storing up problems which could then lead to a tsunami of referrals."
A survey of 1,300 mental-health doctors from across the UK found that 43% had seen a rise in urgent cases while 45% reported a reduction in routine appointments.
One psychiatrist said: "In old-age psychiatry our patients appear to have evaporated, I think people are too fearful to seek help."
Another wrote: "Many of our patients have developed mental disorders as a direct result of the coronavirus disruption - eg social isolation, increased stress, running out of meds."
Dr Bernadka Dubicka, who chairs the faculty of child and adolescent psychiatry at the RCP, said: "We are worried that children and young people with mental illness who may be struggling are not getting the support that they need.
"We need to get the message out that services are still open for business."
Dr Amanda Thompsell, an expert in old-age psychiatry, said using technology to call a doctor during lockdown was difficult for some older people.
They were often "reluctant" to seek help, and their need for mental-health support was likely to be greater than ever, she added.
Mental-health charity Rethink Mental Illness said the concerns raised were supported by evidence from people living with mental illness.
In a survey of 1,000 people, many said their mental health had got worse since the pandemic had started, due to the disruption to routines that keep them safe and well.
"The NHS is doing an incredible job in the most difficult of circumstances, but mental health must be a clear priority, with investment to ensure services can cope with this anticipated surge in demand," said the charity's Danielle Hamm.
She said it could take years for some people to recover from the setbacks.