Eversheds is being investigated by the Occupational Pensions Regulatory Authority (Opra) for failing to make payments into its staff pension scheme.
The criminal investigation could, in theory, land Eversheds in court, with the prospect of unlimited fines and two years' imprisonment for those responsible.
A spokeswoman for Opra says this is the first time a law firm has been reported to the watchdog for such an offence.
The firm admits the payments were missed, but insists this was due to administrative errors that have since been dealt with and that no member of the scheme will suffer financial loss.
Eversheds - which has one of the highest-rated pensions teams in the country - failed six times to pay employees' contributions to trustees.
The trustees of the two funds alerted Opra to the late payments, and Eversheds now faces the prospect of a court appearance. Each missed payment is said to have been between #3,000 and #4,000.
The funds under investigation were previously run by Wilkinson Maugham, the North East firm Eversheds merged with in July 1997.
The missed payments occurred after the merger, when management of the schemes was transferred to a service company linked to Eversheds' Newcastle office.
The Opra spokeswoman confirms that it is conducting a criminal investigation which could lead to Eversheds going to court.
"Like any prosecuting authority, we keep all such matters under review and our board will consider it as and when things arise," the spokeswoman says.
An Eversheds spokeswoman says none of the firm's pension lawyers are involved in the case. "These errors were purely an administrative matter, and were oversights which did not involve any dishonesty or misappropriation of funds," she says
"No member of the pension scheme has suffered any financial loss. Once the errors had been identified they were immediately corrected and new operating procedures put in place in order to ensure no repetition.
"The pension schemes have been operating entirely satisfactorily ever since."