Two police forces have been warned they could face enforcement action over their "disproportionate" use of stop and search powers.
The Equality and Human Rights Commission (EHRC) has written to Thames Valley Police and Leicestershire Constabulary, claiming neither could justify their tactics.
John Wadham, legal director at EHRC, said: "Stop and search needs to be used fairly. The evidence is that it isn't and the commission is acting to try to change this."
The move follows the publication of the commission's report Stop and Think earlier this year, which claimed some police forces were being discriminatory in their use of stop and search powers.
The review, looking at the use of stop and search powers across England and Wales over the past decade, showed black people are six times as likely to be stopped and searched as white people, with Asian people twice as likely to be stopped as white people.
Thames Valley Police and Leicestershire Constabulary were among five forces contacted by the EHRC in May and asked for detailed information after showing "significant and persistent race differences in their use of stop and search", a spokesman for the commission said.
Of the other forces, the Metropolitan Police and Dorset Police have now embarked on the National Police Improvement Agency's Next Steps programme on stop and search powers and are still being monitored by the EHRC. The commission is seeking more information from West Midlands Police before deciding whether to take further action.
The EHRC can exercise powers under the Equality Act 2006 to enforce equality and human rights legislation.
A Thames Valley Police spokesman said: "Thames Valley Police takes this matter very seriously as we are aware of the impact that stop and search can have on diverse communities.
"The EHRC has invited Thames Valley Police to enter into an agreement to ensure steps are taken to avoid a recurrence in the future, and has requested that specific actions be undertaken or ceased.
"The force intends to engage fully with the EHRC in taking this matter forward."
Assistant Chief Constable Steph Morgan, of Leicestershire Constabulary, said: "We are absolutely committed to working towards the elimination of discrimination in the exercise of any of our powers and will work with the EHRC.
"The force is committed to fairness and equality in delivering a policing service to all our communities. Everyone within Leicestershire Constabulary places great importance on these relationships and is aware of the impact that stop and search may have.
"As part of our ongoing aim to deliver the best possible service we have a long history of conducting significant research into the use of stop and search and its impact on the community."