Growing numbers of people who experience a mental health crisis are having to visit A&E or are detained for their own safety because NHS services to help them are deteriorating, MPs and peers have warned.

The 700,000 people in England with severe mental health problems such as schizophrenia and bipolar disorder also face increased delays to get treatment, they said.

“Those who are the sickest often wait the longest to get help,” said a report published on Tuesday by the all-party parliamentary group (APPG) on mental health. Delays make people feel they are “hopeless, and like nothing can help”, it said.

One patient who was unable to access help for their psychosis only received assistance, in the form of “talking therapies”, after their GP told them to lie and downplay the seriousness of their condition, the report said. NHS talking therapy services usually treat only those with anxiety and depression, and not people who have more complex forms of mental illness.

In recent years, core mental health services – particularly community mental health teams and crisis teams – have been overlooked by NHS chiefs, who have prioritised spending on improving treatment in other areas of mental health need, such as support for new mothers and those suffering a first episode of psychosis.

“Faced with rising demand, both of these core services are struggling on their current resource allocation, leading to more people reaching crisis point, attending A&E or being detained under the Mental Health Act,” the report said.

Dr Andrew Moore, a spokesman for the Royal College of Psychiatrists, said: “The neglect of core mental health services, which form the backbone of secondary mental health services, is distressing to see.”

Brian Dow, the deputy chief executive of the charity Rethink Mental Illness, said: “People are being tipped into crisis waiting for treatment. That means more lives are at risk and more strain on the system.”

Core services need to be a key focus of the plans to improve mental healthcare that will feature prominently in the NHS’s forthcoming long-term plan, the APPG believes.

Pressure on NHS services from the growing tide of mental illness means some care is in effect rationed, the APPG inquiry found. “People are being turned away from services or put down to the bottom of waiting lists because they are ‘not sick enough’ for secondary mental health services,” the report said.

For example, some eating disorder services only treat people below a certain body mass index, even if they are already worryingly thin, because specialist nurses and doctors are so overwhelmed.

Care for people in a crisis, perhaps involving suicidal thoughts, self-harm or an attempt to end their life, remain inadequate, despite NHS England’s plans to make them available everywhere by 2021. Every area should operate a 24/7 helpline service for those in acute mental health need to get rapid assistance, the report added. It said the number of people with a psychiatric condition attending A&E had almost doubled since 2009-10.

More positively, the APPG praised the “transformational” impact seen in the four key areas NHS England has made a priority: maternal mental health, early intervention in psychosis, psychiatric liaison services in A&E, and improving access to psychological therapies (talking therapies).

For example, thousands more mothers than originally anticipated are getting specialist help with mental illness related to giving birth.

A cabinet committee should be set up to monitor progress on improving mental health, overseen by the Cabinet Office, the APPG added.