Less than a third of people with common mental health problems get any treatment at all – a situation the nation would not tolerate if they had cancer, according to the incoming president of the Royal College of Psychiatrists.

While the health secretary, Jeremy Hunt, has pledged to deliver "parity of esteem" for mental and physical health services, the treatment gap is now so huge that it may prove impossible to bridge in the current financial climate, said Professor Simon Wessely of King's College London in his first interview since election to the post.

"People are still routinely waiting for – well, we don't really know, but certainly more than 18 weeks, possibly up to two years, for their treatment and that is routine in some parts of the country. Some children aren't getting any treatment at all – literally none. That's what's happening. So although we have the aspiration, the gap is now so big and yet there is no more money," he said.

Wessely said there would be a public outcry if those who went without treatment were cancer patients rather than people with mental health problems. Imagine, he told the Guardian, the reaction if he gave a talk that began: "'So, we have a problem in cancer service at the moment. Only 30% of people with cancer are getting treatment, so 70% of them don't get any treatment for their cancer at all and it's not even recognised."

If he were truly talking about cancer, he said, "you'd be absolutely appalled and you would be screaming from the rooftops."Wessely said he had asked Simon Stevens, the NHS England chief executive, how the gap would be bridged but was told that resolving the issue would involve a "much longer conversation with the public".

"I think what he means is basically, if people really want true parity in the sense of actual 90% of mental health patients treated within 18 weeks, just like they are for other disorders, that is going to have to mean money will have to move from acute to mental health. Genuine money.

"As there is no more money, that would mean significant losses in other sectors. I think he was saying we would need a pretty good political imperative – we would need to know that people were actually on board for that – and I don't know the answer."

A larger proportion of people with psychosis, who have severe mental illnesses such as schizophrenia, are on treatment, but even that figure is still only 65%, according to Wessely, who added: "That doesn't mean they are getting the right treatment or anything like that, but getting something. For most mental disorders it is still the exception not the rule to be recognised, detected and treated. So when we talk about the rise in antidepressant prescribing, before we start leaping to the tumbrils and saying the world's coming to an end we should have a look and say, hang on a second, if that is appropriate prescribing then that's good."

The concern over pills for common mental disorders – for depression, anxiety and attention deficit hyperactivity disorder (ADHD), for instance – could be misplaced, Wessely argued. Much of the criticism assumes that GPs are putting many more people on pills because of an absence of counselling or talking therapies, even though the numbers of therapists being trained to provide cognitive behavioural therapy (CBT) has substantially risen thanks to a government programme called IAPT (improving access to psychological therapies).

Wessely applauded IAPT, but did not accept the argument that talking therapies were necessarily "better", a word that to many people has a moral implication. "If you say they are more effective, I don't really think that's true. I think they are cheaper and easier. CBT is more popular with some people, but other people don't like it," he said. "The truth is most people don't get either. Of course if you are working in areas of high antidepressant prescribing – they tend to be difficult areas like Merthyr Tydfil or Blackpool – of course where you don't have good psychological services then you will use antidepressants. That's not wrong, but what's wrong is you don't have the alternatives."

In the US, the rate of use of stimulant drugs such as Ritalin exceeds the number of people with ADHD, so there is over-medication of the disorder. "But in Britain it is under, which suggests under-prescribing."

The UK is also seriously short of psychiatric beds. "The fact that people are travelling hundreds of miles for a bed, the fact that bed occupancy is now 100% everywhere – in some trusts it is 110% and we're hot-bedding – is a symptom of a system [in crisis]. Relatives and patients hate it. Junior doctors hate it – they spend all their time on the phone. Sometimes bad decisions are made just to get a bed. But we don't think the answer is just let us have some more beds because those will probably fill up as well. We've been told for years that if we just get community care right we won't need beds. That's clearly not true. We will be announcing a commission on beds but it's really on systems. Beds are symptomatic of a problem."

Wessely, who is married to Clare Gerada, recent head of the Royal College of GPs, strongly believes in the need for general doctors, nurses, midwives and social workers to have more mental health training and for there to be much greater integration of diagnosis and treatment of physical and mental disorders. Trials have shown that picking up and treating depression in people with type 2 diabetes improves the control they have over the disease: they take their medication and keep complications at bay. And there are patients who are referred by their GP to a specialist because of a suspected heart complaint – which turns out to be panic attacks that have not been picked up for months.

"The whole of our healthcare system is about separating mental and physical. You couldn't devise a system better suited to separating the mental and the physical if you tried," he said. At King's, psychiatrists have been put into general medical clinics with great results. "Most people have quite complicated views of their illness anyway," he said. They are not resistant to doctors offering cardiac tests and counselling for a recent divorce at the same time.

"Certainly when you look at the cost of investigations, when you look at the cost of treatment that isn't necessary, when you look at the cost of lost working days, when you look at the cost of additional care, actually it does become cost effective. The problem we always have is those savings are not always made to the health service.

"But we know people with physical health problems who also have mental health problems cost about 45% more than those who don't. That's absolutely and unequivocally clear. The cost of their care goes up. They comply less with treatment, they come back more often, they have lower satisfaction and they have more complications."

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