Ministers fear an autumnal surge in swine flu could cause health service staff shortages of up to 25 per cent.

In response, the department of health is, until October 7, consulting on plans to temporarily reduce from two to one the number of psychiatrists required to detain patients for assessment or treatment under mental health law.

The emergency plans include suspending requirements for a second-opinion appointed doctor to approve medicating a patient without consent if they have been in hospital for three months or more.

The plans also include allowing retired approved social workers to be designated approved mental health professionals.

Sir Liam Donaldson, chief medical officer, said the temporary changes were to maintain levels of care while protecting the safety and rights of patients and the public.

But campaigners and psychiatrists voiced concerns.

Mind's chief executive, Paul Farmer, said: "Any proposals that will reduce the number of professionals involved in the sectioning process are concerning.

"Sectioning effectively deprives people of their liberty, and the reason that a number of professionals are involved is to ensure that the best decision is made for the patient, and no one is detained inappropriately.

"We have to be clear that these are serious changes, and should only be used as a last resort.

Professor Louis Appleby, the government's national director for mental health, said: "We are determined to make sure we have a sensible, proportionate approach that ensures that vulnerable mental health patients continue to get the treatment they need, when they need it, even in the event of staff absences."