Cameron McKenna is being taken to an industrial tribunal on grounds of race discrimination by a former lawyer in its CIS group.
Natasha Lobova, a Russian-qualified lawyer based in London, left the firm last July.
Her claim is at least the second claim to be brought against Cameron McKenna by a member of the CIS group in the last 18 months.
Last year a secretary in the group, Veronica Pescud, claimed disability discrimination and sex discrimination against the firm. It is understood the case was settled out of court last August.
The news comes as two other Russian qualified assistants leave the CIS group. Dmitri Chebotarev and Natasha Thompson departed last Thursday.
Cameron McKenna refused to comment on Lobova's case when The Lawyer went to press.