Mr Fox, who was Tory health spokesman before becoming shadow foreign secretary, will say that a humane society should not tolerate such a failure in government policy and that such poor levels of care would not be tolerated for other types of illness.

This comes as ministers still face opposition to mental health reforms that are expected to be announced in the Queen's Speech. Officials have made some amendments to the controversial draft Mental Health Bill but campaigners say that the proposed laws are still biased heavily in favour of compulsion rather than care.

The Independent on Sunday has been campaigning for the rights of mental health patients to be respected by the Government. The paper has highlighted the plight of those in secure hospitals who are eligible for transfer but are stuck in the system because of a lack of beds in more suitable psychiatric units.

In his speech at the Sane mental health charity meeting, Mr Fox will say women bear the brunt of the burden of mental illness because they have to provide care when the system fails, and that many people end up sleeping rough because there is no adequate support.

He will say: "When we... see people, many of whom will suffer from a mental illness, sleeping in the doorways of some of our most beautiful buildings... then we are witnessing a policy failure that a humane society should not tolerate."