A survey by the College found almost half (45 per cent) of psychiatrists had seen a reduction in routine mental health appointments, leading to fears that patients were avoiding support until they reach crisis point.
Meanwhile, 43 per cent of psychiatrists had seen an increase in their urgent and emergency caseloads, where patients were showing the most serious conditions.
Professor Wendy Burn, president of the Royal College of Psychiatrists said: “We are already seeing the devastating impact of Covid-19 on mental health with more people in crisis.
“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.”
The pandemic has made it “much harder” for mental health services to offer routine appointments, despite the introduction of remote consultations, the college said.
Its survey of more than 1,300 mental health doctors in the UK suggests the biggest reduction in routine care had been for older adults, children and young people, as well as within general hospitals.
The college said steps must be now be taken to ensure mental health services are ready to help people as the number of coronavirus cases begins to fall.
Prof Burn added: “Mental health services will be at risk of being overwhelmed unless we see continued investment.”
The Office for National Statistics found that between March 20 and March 30 almost half (49.6) of people in Great Britain reported levels of anxiety.
Meanwhile, research by the Royal Society for Public Health found that young people were more likely to experience poor mental health and wellbeing under lockdown than older adults.
In regard to older people, Dr Amanda Thompsell, chairwoman of the faculty of old age psychiatry, said they are “often reluctant” to seek help, while their need for mental health support is “likely to be greater than ever”.
She said: “We are worried about the impact of shielding and self-isolation, anxiety about the virus and the difficulty some older people find in using technology to video-call a doctor.
Written responses to the survey from psychiatrists suggested that some older patients were “too fearful” to seek help.
Others reported that patients had more severe psychotic symptoms which incorporated Covid-19 themes.
Dr Jim Bolton, chairman of the faculty of liaison psychiatry, said: “Following a quieter than normal period, we are now seeing more vulnerable patients presenting in crisis.
“Many of these patients have suicidal thoughts or have harmed themselves.
“The pandemic is having a serious negative impact on people with mental illness and we are worried things could get worse.”
A total of 1,369 psychiatrists responded to the College’s survey between May 1 and May 6.
Emma Thomas, chief executive of the mental health charity YoungMinds, said: “We know that many young people have lost or had their support reduced, and this [survey] confirms that many are not seeking help until they reach crisis point, with potentially devastating consequences.”
An NHS spokeswoman said mental health services continue to be “open and available”, adding that they have been adapted to ensure people can still receive therapy and counselling from their clinician.