Which lawyer would be prepared to act for Josef Fritzl — accused of locking his daughter up for 24 years, repeatedly raping her and fathering seven children by her?

Barristers in England and Wales vaunt the cab-rank principle as a badge of honour: the rule that in theory requires a barrister to take the next case that comes along, whatever it is and however distasteful.

Even the most heinous killers — Fred and Rosemary West, Peter Sutcliffe, the Moors murderers Myra Hindley and Ian Brady — all found barristers to take their cases.

The standard dinner party question for barristers is: how can you act for someone you know or suspect must be guilty?

One answer is that they do not know, because the accused has not — until after trial — been found guilty beyond reasonable doubt. And if they suspect, it is irrelevant: because even a Josef Fritzl is entitled, under our legal system, to be properly represented; to have his case put as best as it can be put, with the burden on the prosecution to prove its case.

But another answer is the cab-rank principle. This means that a Jewish barrister would act for someone accused of war crimes, an ardent feminist for a serial rapist or a black barrister for a member of the BNP. That does happen: many advocates pride themselves on taking cases on both sides of the fence and on not being solely prosecution or defence briefs.

There was a furore when Cherie Booth, QC, acted for the Crown against a poll-tax defaulter at the time Tony Blair was Labour leader but not Prime Minister. Similarly, Lord Neill of Bladen, QC, came under fire when — arguing the cab-rank rule — he agreed to act for Dame Shirley Porter, the Conservative leader of Westminster Council, in her appeal against being fined £23 million after being found guilty of wilful misconduct.

The idea behind the principle is that anyone in the land can have access to the best brains at the Bar. Solicitors in remote corners of the British Isles can instruct leading counsel of their choice in London, Birmingham or Manchester.

In practice, though, the rule is more honoured in the breach than in observance. Sought-after barristers can always plead being booked — and can pick and choose their cases with impunity. Solicitors will also pick horses for courses: they might go, for instance, to Edward Fitzgerald, QC, to argue a case on prisoners’ rights. And there is the question, increasingly relevant in these days of specialisation, of expertise. The numbers of barristers who have expertise and experience in a given area of law may be few.

Family barristers, when in dispute over the low levels of legal aid fees, also opened a fresh chink in the cab-rank principle when in 2001 they said that barristers need accept work only at a fair level of remuneration.

So when can a barrister turn down a brief? Barristers are not meant to withhold their services on the ground that the case is “objectionable” to him or her or to any section of the public; or that the client’s beliefs are unacceptable or because of the source of financing provided to the client. But there are wide grounds for refusal: they must turn down work if they lack experience of competence in a case; and can do if they will not have time to prepare the case properly, for instance. Or if the fee is not adequate to the case.

The Bar Standards Board, the body that regulates barristers in England and Wales, has just published a consultation paper, launching debate on when barristers can decline to be instructed on a case or withdraw from it.

Ruth Evans, chairman of the board, said: “It is in the public interest to ensure that cases are able to proceed with certainty; the changes are designed to clarify the circumstances in which a barristers may withdraw from a case and where a barrister is obliged to provide representation.”

Changes in the way barristers may practice — with more in the pipeline under the Legal Services Act — make the time ripe to review the rule again. The argument about access to counsel of choice has been gradually weakened — by, for increasing numbers of solicitor-advocates and in-house barristers being able to appear in the higher courts for their employers. Partnership between barristers and solicitors looms — and with it issues of conflicts of interest.

The cab-rank rule, in this context, may be an unjustified restriction on barristers, preventing them openly choosing what cases they do. It goes against the trend towards removing prohibitions on how lawyers can practise. But worse, it is not what it seems. Members of the public cannot have any barrister they want; and barristers need not take the next case that comes along.

The rule could be scrapped altogether. But that would be an over-reaction to the changing legal landscape. Better to have it redefined, so that all freelance, self-employed advocates — whether solicitors or barristers — are bound by it. And to ensure that the rules for turning a brief away are clear and rigorously enforced. Otherwise the Bar will lose one of its best and most fundamental tenets (not to mention best marketing points) and more important, the Josef Fritzls could go unrepresented.

Times Recommends

• The best legal yarns this Christmas
• Legal school 'earned £1m by exploiting bar course'
• Axe may fall on 5,000 more City lawyers