Hickman & Rose, which is investigating allegations of brutality at Wormwood Scrubs, has accused officers at the jail of pressing prisoner clients to dump the firm.
The accusation follows senior partner Jane Hickman's arrest on suspicion of supplying drugs while visiting a client at Wandsworth prison earlier this month. It is contained in an open letter by the firm to the Home Secretary, Jack Straw.
In it, partner Daniel Machover speculates that some officers at Wormwood Scrubs may be attempting to capitalise on Hickman's arrest by discrediting the firm with suggestions that it had "given drugs to prisoners in return for making allegations of brutality".
The Lawyer understands that one client who has made an allegation of brutality at Wormwood Scrubs has already stopped instructing the firm because of Hickman's arrest.
In a letter to The Lawyer expressing her innocence, Hickman said: "It would be equally wrong to leap to any conclusion as to who, other than I, might have been the source of the drugs. However, the issue appears to be diverting attention from the real issue, which is brutality."
A spokesman for the Prison Service said the service was co-operating with the police in its investigation of the prisoners' original claims but the new allegations needed to be substantiated before "any other inquiry is considered".
He added: "We have taken a number of steps to protect those who have made allegations and will consider any other measures the police may feel are necessary."