Those who would suffer include children and people with eating disorders.

The warning was given during a meeting of the Scottish Parliament's Finance Committee

MSPs were looking at the funding available for a new bill backed by the Scottish Executive.

Clearly the bill will not work properly unless enough is provided for the necessary number of mental welfare officers

Dr Jim Dyer Mental Welfare Commission

The Mental Health (Scotland) Bill, launched in September, promised the most radical reform of the service for 40 years.

Dr Jim Dyer, director of the Mental Welfare Commission for Scotland, said there would have to be "a very significant expansion" of mental welfare officers and consultants.

He said: "Clearly the bill will not work properly unless enough is provided for the necessary number of mental welfare officers who have specific responsibilities under the bill."

He said the Royal College of Psychiatry had estimated the bill would create a need for between 18 and 28 more consultants.

One in five Scots has a psychiatric problem

However, 29 consultancy posts were empty in Scotland last month.

Dr Dyer said: "There are some areas of the service where there are unacceptably long waiting lists."

He said this was a problem in children's services, for people with eating disorders and those needing to see psychologists.

He said: "Clearly these are area difficulties which may well need to be made up if the aspirations of the bill are going to be properly implemented," said Dr Dyer.

Dr Lindsay Burley, chair of the NHS Scotland Board chief executives' group, said the bill would place particular stress on psychiatrists and mental health officers.

Pay review

She added: "There is a concern that the tribunal system may pull staff, already stretched and in short supply, away from the service."

Jim Brown, head of the Scottish Executive's public health division, said those in charge of the bill were considering the potential workforce problems "very carefully".

He said: "We are aware of the potential pitfalls and the implications to be considered, both in terms of workloads for psychiatrists and mental health officers."

Mr Brown said the executive was already carrying out a review of pay structures across the NHS and that a special group had been set up to look specifically at mental health staff.