Around 6% of public tip-offs about illegal immigration lead to an investigation and only 1.5% results in a person being removed from the country, according to a Home Affairs Select Committee report.
The report also said the now-defunct UK Border Agency (UKBA) had a backlog of 432,029 immigration and asylum cases when it was scrapped at the end of March, which at current levels will take five years to clear.
The Metropolitan Police had full access to all material relating to the Serious Organised Crime Agency's (Soca's) investigation into blagging of private details, which led to the jailing of four private investigators, the agency said.
A spokesman for the Met confirmed detectives received all of Soca's information from Operation Millipede between May and December 2011, but said:
This was essentially huge quantities of electronic data, the hard drives of which were supplied to the MPS.
This material was only put into list format for the Home Affairs Select Committee, and the MPS only provided with the information in that format on July 30.
The statement from the Met added:
In common with Soca, the MPS is not alleging that the individuals or companies on the list have, or even may have, committed a criminal offence.
The MPS is checking the list against its open investigations and will inform the committee when this is complete.
House of Commons Home Affairs Committee chairman Keith Vaz has said it is "very surprising" that the list of firms linked to rogue private investigators was not shared with the Met given the investigations into alleged hacking.
Detectives are carrying out inquiries codenamed Weeting, Elvedon and Tuleta into allegations of phone-hacking, illicit payments to public officials and computer-hacking by journalists, some linked to private investigators.
The Serious Organised Crime Agency (Soca) provided Mr Vaz's committee with the list - containing 102 firms and individuals, including blue chip companies, law firms and financial services groups - on condition that the names are not revealed.
But following the shock resignation of Soca chairman Sir Ian Andrews, Mr Vaz has said he will ask his successor to review the decision. He today said that Soca should act with "urgency", arguing that further delay would be unfair to those on the list who want the affair cleared up.
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.