HM Revenue and Customs has confirmed that a further six data discs have gone missing in transit between its offices in Preston and London.
Police have finished their search for the discs at HMRC offices
The discs, which were reported missing on 30 October, contained recorded conversations between a member of staff and a customer making a complaint.
Police are still searching for two computer discs containing the details of 25m Child Benefit claimants.
The HMRC says evidence suggests these two discs are still on its premises.
The second lost package, containing six discs, went missing after being sent from a tax credit office in Preston to HMRC's Whitehall headquarters in London.
They were despatched through the same internal mail system used by those who sent the two missing Child Benefit discs, which have not been seen since being posted at HMRC in Washington, Tyne and Wear, on 18 October.
Police are now focusing their search for the lost Child Benefit records discs on depots of the courier company TNT after completing inquiries at HMRC's offices.
A spokesman for TNT confirmed its premises in London had been searched on Friday night and said the company was fully co-operating with police.
But he added that it was impossible to say whether the CDs had entered TNT's system.
"We have been given absolutely no proof either from HMRC or from the police that these discs ever entered the TNT system, let alone that we have mislaid them.
"HMRC confirmed to us last night that they use two other companies for internal mail."
An HMRC spokesman confirmed the agency did use other courier services and said that the hunt for the discs would be "wide-ranging and comprehensive".
"All the evidence points to the fact that these discs are still on our premises," he added.
Police looking for the discs completed their search at the Washington HMRC building on Friday night.
A Scotland Yard spokeswoman confirmed searches had now started at TNT buildings, but would not say at how many or reveal the locations.
A core team of 47 detectives from the Specialist and Economic Crime Directorate has been involved in the search operation.
The first two missing discs hold the personal details of all families in the UK with a child under 16.
The data on them includes the name, address, date of birth, National Insurance number and, where relevant, bank details of 25 million people.
The discs were intended for the National Audit Office (NAO) in London, but never arrived from HMRC's office in Washington, Tyne and Wear.
In March, officials at HMRC began the practice of downloading the entire Child Benefit database onto CDs and sending them through the internal mail to the NAO for auditing.
The practice came to light only on Tuesday when it was revealed the two discs had been lost - raising fears that data protection laws had been broken.
On Friday, it also emerged that the NAO had handed unencrypted discs containing details of all Child Benefit claimants to accountants KPMG.
The NAO said they had been delivered by hand and returned safely.
Meanwhile, a row has broken out between the government and the banks over who will pay the cost of any resulting fraud from the loss of the two Child Benefit discs.
Both the chancellor and the prime minister told MPs the banks would repay customers who lost money.
But in a letter, signed by the British Bankers' Association, the Building Societies Association and the Payment Service APACS which is responsible for security of money transfers, the banks have told the chancellor that he should reimburse them for the cost.
The Treasury has not yet responded.
Those affected by the loss of the discs are being asked to check their bank accounts for any "irregular activity", but have been told there is no need to close accounts as the details would not be sufficient to allow fraudsters to access them.
However, people have been advised not to give out personal or account details "requested unexpectedly" by phone or by email.
LOST CDS - SEQUENCE OF EVENTS
18 October - Junior official from HMRC in Washington, Tyne and Wear, sends two CDs containing password-protected records to audit office in London through courier TNT, neither recorded nor registered
24 October - When package fails to arrive, second one is sent by registered post and arrives safely
3 November - Senior managers are told first package has been lost
10 November - Prime minister and other ministers are informed
12 November - HMRC tell ministers CDs will probably be found
14 November - When HMRC searches fail, Metropolitan Police are called in
15 November- Richard Thomas, Information Commissioner, says remedial action must be taken before public is informed
20 November - HMRC Chairman Paul Gray resigns; Chancellor Alistair Darling makes announcement to House of Commons
21 November - Prime Minister Gordon Brown apologises and orders security checks