Legal Services Ombudsman Ann Abraham told the Office for the Supervision of Solicitors (OSS), the professional body which investigates complaints, that it is allowing the situation to "spiral out of control".
She said there was currently a backlog of about 17,000 complaints which meant that the OSS could take six months to deal with a standard letter about poor service.
In her eighth annual report, published on Wednesday, Ms Abraham told the OSS, which is the complaints-handling arm of the Law Society, that it had missed its own published standards and targets.
And she repeated warnings made in the previous year's report, that the OSS must improve its performance or face losing its role.
Ms Abraham warned that the government could take over the complaints process should it judge that industry self-regulation was failing.
1,466 investigations
"This is a sharp reminder that self-regulation within the legal profession is a privilege, not a right - and a privilege that can be taken away," she said.
In the 15 months to 31 March 1999, one complaint was made for every 2.4 practising solicitors, compared with one complaint for every 18 practising barristers.
The number of cases being investigated by her office because complainants were not satisfied with the way the OSS had dealt with them reached an all-time high of 1,466 during that time, she said.
The Law Society admitted that the delays in dealing with complaints were "unacceptable", but said that it had started to address the problem.
Earlier this month, it decided to invest £5.7m in the complaints system, and to recruit an additional 23 permanent caseworkers and 82 temporary caseworkers.
This should reduce the time taken to deal with complaints to three months at the most, it said.
Ms Abraham welcomed the move, but said: "It is a serious indictment of the Law Society that they have allowed things to reach such a crisis before taking action."
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Crackdown on asylum law firms
(17 Feb 99 | UK)
'Rude' barristers advised on politeness
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