Ed Miliband is to launch a broadside at celebrities who make light of mental illness, as he unveils plans to tackle what he terms "the biggest unaddressed health challenge of our age".
The Labour leader will criticise writers and TV personalities Jeremy Clarkson and Janet Street-Porter for articles which he claims insulted and belittled people with mental illness and contributed to a national taboo on the issue.
Many people in Britain could get treatment for mental problems but are "intimidated" from seeking help by the fear of being mocked and subjected to "lazy caricatures", he will say.
Failure to tackle the taboos and address mental health issues is blighting the lives of millions of people, adding £10 billion to annual NHS spending and costing business £26 billion a year in reduced productivity, sickness absence and the cost of replacing staff who can no longer work, he will warn.
In a high-profile speech in London, Miliband will say: "One in four of us will have a mental illness at some point in our lifetime. It is the biggest unaddressed health challenge of our age.
"There are so many people in Britain today who could be treated but who are intimidated from seeking help. And so many people who need support but who believe that no one will care.
"For far too long our leading politicians have been far too silent about mental health, part of a taboo running across our society which infects both our culture and our politics.
"It is a taboo which not only blights the lives of millions but also puts severe strain on the funding of our NHS and threatens Britain's ability to pay our way in the world. It is a taboo which must be broken if we are to rebuild Britain as one nation.
"There are still people who abuse the privilege of their celebrity to insult, demean and belittle others, such as when Janet Street-Porter says that depression is 'the latest must-have accessory' promoted by the 'misery movement'.
"Jeremy Clarkson at least acknowledges the tragedy of people who end their own life but then goes on to dismiss them as 'Johnny Suicides' whose bodies should be left on train tracks rather than delay journeys.
"Just as we joined the fight against racism, against sexism and against homophobia, so we should join the fight against this form of intolerance. It is not acceptable, it costs Britain dear and it has to change."
Miliband will announce the creation of a taskforce headed by the chair of Barts NHS Trust, Stephen O'Brien, to draw up a strategic plan for mental health in society.
And he will promise to rewrite the NHS constitution to give patients the same legal right to therapies for treating mental illness as they already have to drugs and treatments for physical illness.
Labour would ensure that training for all professional staff covers mental health and would integrate physical and mental healthcare more effectively, he will say.
In his first major speech since he claimed the One Nation mantle of Benjamin Disraeli for Labour, Miliband will argue that Britain cannot be one nation if it writes off people with mental illness.
Just as Disraeli tackled the issue of sanitation to improve the nation's health in the 19th century and Labour created the NHS to deal with physical health in the 20th, it is now time to address the issue of mental health in the 21st, he will argue.
"Like sanitation, this is a massive public health challenge affecting millions," Miliband will say. "Like the demand for an NHS, it reflects how our national response is so inadequate against the scale of the challenge.
"In both cases, sanitation and the NHS, it took war to shock us out of complacency. This time we can't wait for greater crisis. We must act. It's about changing the way we do things so we save money in the long term.
"The last Labour government began to transform mental health provision. It made well-respected, evidenced-based therapies available to more people than ever before. Taking mental health treatment into communities that had never received them before. We need to do all we can now to protect those programmes now."
He will also argue that poor mental health cannot simply be solved by government schemes but will require changes in the workplace, communities and schools to support those affected.
"In fact, everybody has a part to play. Only a nation acting together can overcome the challenge we face," he will say.
Miliband has criticised Clarkson in the past for having a "‘deeply pessimistic" view of national identity after the Top Gear presenter used a newspaper column to bid "good riddance" if Scottish people wanted to leave the union.