Two missing computer discs containing the personal details of 25 million people could be worth up to £1.5bn to criminals, say the Lib Dems.
Mr Cable said the discs could dwarf the value of the Brinks Mat robbery
Acting leader Vincent Cable told MPs an "enormous amount" was still at stake, after discs containing the entire child benefit database got lost in transit.
Ministers say there is no evidence they have been intercepted by criminals.
But in a Tory-led debate on the issue shadow chancellor George Osborne asked if the "whole truth" had been told.
The discs were lost after HM Revenue and Customs sent discs to the National Audit Office unregistered and unencrypted.
Prime Minister Gordon Brown and Chancellor Alistair Darling have apologised for the missing discs, which were said to have been sent, via courier TNT, to the NAO offices in London on 18 October.
While they say there is no evidence the discs have fallen into the wrong hands, millions of families have been told to be on alert for attempts to fraudulently use their details - which include addresses, bank account and National Insurance numbers, as well as children's names and dates of birth.
During the Opposition Day debate, Mr Cable said he sincerely hoped the discs would not fall into the hands of "the criminal fraternity".
He said he understood that on the black market one identity was worth "something in the order of £60".
He said the entire database had "a criminal value of something in the order of £1.5bn which makes the Brinks Mat robbery rather the equivalent of robbing the church collection by comparison, if the criminal world were to get hold of it".
But Mr Darling told MPs: "The police inform me that they still have no evidence or intelligence that this data has fallen into the wrong hands and no evidence of fraud or criminal activity."
He said that security changes had been made at HMRC so that "bulk data transfers" would now only be made when "absolutely necessary" and with written authorisation by senior managers.
He added that "clear instruction" would have to be given regarding the protection of such a transfer.
Meanwhile shadow chancellor George Osborne urged Mr Darling to provide evidence to back up the claim being sent to child benefit recipients that "the copy of the data is likely to still be on government property ".
And he accused Mr Darling of "not telling the British public the whole truth about how their personal details came to be lost".
He said, while Mr Darling's Commons statement last week implied "it was all the fault of ... a junior official at HMRC" that was "not close to an accurate statement of what actually happened".
Emails released by the National Audit Office showed that senior officials at HMRC had been involved in the decision to release the database to the NAO, he said.
But Mr Darling denied this and said the exact details of what had happened is the subject of an inquiry by Price Waterhouse Coopers chairman Keiran Poynter and the NAO.
"They will examine the evidence, establish the facts and then make recommendations."