The Metropolitan Police did not see a controversial list of 102 firms and individuals linked to rogue private investigators until last week, despite having access to the operational information that was used to compile it.
Chairman Keith Vaz said the Home Affairs Select Committee "remains concerned" after it was handed a list of 102 names of companies and individuals linked to rogue private investigators.
The Committee remains concerned that it holds a list that Soca [Serious Organised Crime Agency] has classified as secret, even though it is evidence given as part of our inquiry.
This is an important step forward in establishing the facts.
Mr Vaz also confirmed there are five organisations or individuals on the list who are being investigated as part of Operation Tuleta, Scotland Yard's investigation into allegations of computer hacking.
Labour MP Keith Vaz has written to the Serious Organised Crime Agency (Soca) for answers over a list of 102 names of companies and individuals who allegedly hired private investigators to collect confidential data.
The chairman of the Commons Home Affairs Select Committee wants to know who authorised for the list to be classed as confidential.
In his letter to the director general of Soca, the MP said: "You must be aware that the companies and individuals detailed as part of your investigation impact on people's everyday lives and therefore, there is a legitimate public interest in who made, and on what grounds, the decision to withhold."
In response, Trevor Pearce said there was "scope" for future investigations, telling Mr Vaz that remained the reason for not making the list public.