Civil liberties firm Bindman & Partners has asked all the chambers it instructs for an assurance they are complying with equal opportunities legislation as well as a list of its ethnic minority and women barristers.
Letters making the request have been sent out to 50 chambers and will be viewed by some sets as a warning that they must either improve their equal opportunities record or lose Bindmans' work.
"Ethnic minority and women barristers have difficulty obtaining their fair share of good quality work," the letter says.
A partner at the firm said: "To say it's a threat is a bit unsubtle. But we are anxious that we deal only with those sets with good equal opportunities."
Senior partner Geoffrey Bindman said: "We try to monitor instructions sent out to counsel to ensure it is fairly distributed. This is just a natural development of that policy."
The move coincides with increasing pressure on the Bar to comply with equal opportunities law. Last week Lord Irvine sent a letter to all heads of chambers asking for their help in encouraging ethnic minority lawyers to apply for silk and to the bench.
Lincoln Crawford, head of the Bar Council's race relations committee who approved the text of the Bindman & Partners letter, welcomed the initiatives. He said: "Chambers can no longer duck responsibility over issues of equality."