The lawyers representing four child abusers who are fighting deportation are being investigated by the Solicitors Regulation Authority (SRA).
Burton & Burton were criticised by the presiding judge during a delayed immigration tribunal for the men, who were convicted of crimes in Rochdale, Greater Manchester, in 2012.
Mr Justice Bernard McCloskey said they had shown "frankly shameful" behaviour.
The Nottingham-based solicitors has yet to comment on the investigation.
The SRA said Mr Justice McCloskey's comments were of "real concern".
The judge, who is president of the Immigration and Asylum Chamber, said in February that the men's legal team had failed to submit the necessary papers to the court and repeatedly asked for adjournments.
He added that their conduct had been "cavalier and unprofessional".
The tribunal ended with the four losing their legal battle to maintain British citizenship.
'Waited long enough'
Rochdale MP Simon Danczuk, who complained to the SRA, said the solicitors had "prevented criminal foreign nationals returning to their country of origin".
"The victims of the crimes committed by the four men in question must not be forgotten," he said.
"They have waited long enough for justice, and it is only right... the perpetrators of these crimes now serve their time in a Pakistani jail."
The SRA said it has the powers to deal with fitness to practise issues and can issue fines and close firms down in the most serious cases.
It can also report cases to the Solicitors Disciplinary Tribunal, which has the power to strike off individual lawyers.
The four men were among nine convicted of offences against girls as young as 13.
The gang's ringleader Shabir Ahmed was given a 19-year sentence for offences, including rape, and two months later, he was handed a 22-year concurrent sentence for raping another child 30 times over the period of a decade.
Abdul Aziz, Adil Khan and Abdul Rauf were also jailed for child sex offences, for nine, eight and six years respectively.