The Information Commissioner is to look into whether a Conservative Justice Minister breached regulations on conduct by hiring private detectives.
Jonathan Djanogly, MP for Huntingdon, admitted last year that he had paid detectives more than £5,000 to spy on local party members because he was concerned they were conspiring against him. He said later that he "over-reacted" but denied that he had done anything wrong.
However Labour MP John Mann has now written to the ICO accusing Mr Djanogly of using "improper methods" and "wholly unethical" practices.
Mr Mann said the minister should resign because he no longer had any credibility. "He can't be responsible for the courts if he is also responsible for people who could be up in front of them," he said.
The ICO said there was on ongoing inquiry into the matter.
Mr Djanogly, a member of New London Synagogue, is the son of Nottingham businessman and philanthropist Sir Harry Djanogly. He was elected to Huntingdon in 2001, taking the seat of John Major.
In a statement he again denied condoning anything "unlawful and dishonest in the investigations". He said the investigators had assured him they only acted lawfully and said the ICO had no relevance in the matter.