In English law (and other countries which adopt the rule) the expression cab rank rule refers to the obligation of a barrister to accept any work in a field in which he professes himself competent to practise, at a court at which he normally appears and at his usual rates, also known as when you have to take the first black cab, in the row.
Paragraph 602 of the Code of Conduct of the English Bar states:
A self-employed barrister must comply with the ‘Cab-rank rule’ and accordingly except only as otherwise provided in paragraphs 603 604 605 and 606 he must in any field in which he professes to practise in relation to work appropriate to his experience and seniority and irrespective of whether his client is paying privately or is publicly funded:
(a) accept any brief to appear before a Court in which he professes to practise;
(b) accept any instructions;
(c) act for any person on whose behalf he is instructed;
and do so irrespective of (i) the party on whose behalf he is instructed (ii) the nature of the case and (iii) any belief or opinion which he may have formed as to the character reputation cause conduct guilt or innocence of that person.
In the absence of such a rule it might be difficult for an unpopular person to obtain legal representation, and barristers who act for unpopular persons might be criticised for doing so.
In 1999, Lord Irvine (then Lord Chancellor) said: “The “cab rank” rule is one of the glories of the Bar. It underscores that every member of the Bar is obliged, without fear or favour, to represent clients who offer themselves, regardless of how unpopular they may be in the community or elsewhere.”
The cab rank rule is not universally admired and in 2010 the Law Society of England and Wales said: “The Society questions whether the cab-rank rule remains a necessary and proportionate rule for the Bar at a time when there is increasing competition for advocacy services.” (source – Wiki)