A leading neuroscience expert from Cardiff University says more needs to be invested in mental health research.
Professor Mike Owen will also tell AMs that more needs to be done to change attitudes and stigmas surrounging mental illness and disability.
He says just 5% of medical research is in the field of mental health despite it affecting 16.7m people in the UK.
The assembly government says mental health services are a priority, with hundreds of millions ring-fenced.
His call has been backed by mental health charities SANE and Mind Cymru.
Professor Owen said mental ill health cost the UK economy around £77bn per year, with dementia costing £23bn per year.
Earlier this year the All Wales Mental Health Promotion Network put the cost of dealing with mental health problems in Wales at £7.2bn a year.
Prof Owen, director of Cardiff University's new neuroscience and mental health research institute, says strategies for funding research must be overhauled.
"The development of new therapies for severe disorders such as schizophrenia, bipolar disorder, Alzheimer's disease and ADHD is severely hampered by our lack of understanding of disease mechanisms," he said.
Professor Owen will explain to assembly members how 15% of disability resulting from disease is due to mental illness.
"These dwarf figures for any other class of disease, and mental illness is the leading cause of disability in Europe," he said.
"There is no indication that this situation is improving and as the average age of the population increases it is likely to worsen."
Having worked in the field of mental illness for more than 20 years, he believes that the stigma surrounding the illness is the main reason for research being "grossly underfunded".
"As well as resources, the 'War on Mental Illness' will require a change in mindset for many," he added.
"These severe disorders are brain diseases which are tractable to scientific research and which will ultimately be treatable.
"We will also need to include more patients, carers and health professionals in research and make sure that we realise the largely untapped potential of the NHS to contribute to this effort."
Marjorie Wallace, chief executive of SANE, said the mental health charity had been campaigning for more funding for research for 25 years.
"I tend to agree with Professor Owen - it is time we went to war on mental illness," she said.
"Our belief is that until we find out the causes of mental illness, which we do not yet know, we will never be able to erase the stigma surrounding them."
Meanwhile, a spokeswoman for Mind Cymru called for a coordinated effort to break down stigma and discrimination towards people with experience of mental distress.
A Welsh Assembly Government spokesperson said it is providing funding of £827,000 to the Mental Health Research Network Cyrmu and £733,000 to the Wales Dementias and Neurodegenerative Diseases Research Network over the next five years.
It also co-funds the Medical Research Council centre for neuropsychiatics and genetics at Cardiff University and independent researchers.
"Mental health services are a key priority for the assembly government - as demonstrated by more than £572 million being ring fenced 2010/11 and we are providing an additional £1.5m a year to extend and develop dementia care provision, to meet the anticipated increase in demand in the future."