The Metropolitan Police has said it did not see a controversial list of 102 firms and individuals linked to rogue private investigators until last week.
The list was compiled by the Serious Organised Crime Agency (Soca), which is coming under pressure from MPs to publish the names of those found to have used investigators to obtain private information.
The Met confirmed it had access since 2011 to the information compiled by Soca that led to the jailing of four private investigators and was the basis for the agency's compiled list.
But a spokesman said police had not seen that list until last Tuesday, after it had been confidentially submitted to the House of Commons Home Affairs Committee.
The Home Secretary announces that private investigators will be regulated. They will now have to be licensed, including training and criminal checks. They could face prison and a fine if they operate without a licence.
Speaking at a news conference, Nick Clegg said he is very concerned about rogue private investigators. He told reporters, he will be publishing plans today to bring private investigators under the umbrella of regulations in place for the rest of the security industry.
Chairman of the Commons Home Affairs Select Committee, Keith Vaz, says questions need to be answered about why personal data was illegally obtained by more than 100 firms and what the information was used for.
He said: "What did they do when they got this information? Some of the information we received on the committee showed that they were getting the most personal details from people. Why would they want all this information? I think that's the next stage."
Mark Lewis, the solicitor for the parents of Milly Dowler, has said it is "not surprising" the Serious Organised Crime Agency has discovered 102 companies, including 22 law firms, had commissioned work from four private investigators convicted of illegally obtaining private information:
Home Affairs Select Committee chairman Keith Vaz has written to the Met Police's Assistant Commissioner Cressida Dick to ask if the force has any further interest in the companies on a list of names linked to rogue private investigators.
He has asked Ms Dick, as well as the Information Commissioner's Office, to confirm when the public will be able to see those names, whose identities are currently being protect at the behest of the Serious Organised Crime Agency (Soca.)
The list, which was created by Soca, was passed to the Home Affairs Select Committee on strict confidentiality grounds - prompting Mr Vaz to demand an explanation as to why it should be kept secret.
It breaks down firms that featured in evidence in prosecutions of Operation Millipede, the Soca investigation that led to the conviction of private detectives for fraud, as well as firms that were relevant to the inquiry but not used in evidence.
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.