Two people have been arrested on suspicion of falsely countersigning more than 30 UK passport applications.
Twelve fraudulent identities were allegedly uncovered
A man, aged 27, from Tooting and a woman, 41, from New Cross, both in south London, were detained over immigration and forgery offences.
They were held after police raided 11 addresses following a six-month operation targeting immigration crime.
Another woman, aged 39, was also held on suspicion of helping illegal immigrants in and out of the UK.
'Risk of abuse'
In the past seven months, 14 other people have been arrested over immigration related offences as part of the operation.
Police have identified 12 possible fraudulent identities, including some alleged impostors using more than one identity.
The inquiry began after the Identity and Passport Service received a number of passport applications countersigned by the same two people.
Det Ch Insp Nick Downing, who headed the operation, said: "This investigation has highlighted the potential abuse of the counter-signatory system."
There are strict rules covering the countersigning of passport applications. and the maximum sentence for facilitation of illegal immigrants into and out of the UK is 14 years.
Bernard Herdan, of the IPS, said new vetting procedures, including face-to-face interviews, will "deliver a step change in our ability to combat the fraudsters and forgers".