The Tories claimed the emails cast fresh doubt on the Chancellor's claims that the loss of the database was down to a junior official in the HM Revenue and Customs (HMRC) offices in Washington, Tyne and Wear, who ignored proper procedures.

The emails also reveal that the National Audit Office (NAO) explicitly urged HMRC to ensure that the data was delivered "as safely as possible due to their content".

Mr Darling's Commons statement on Tuesday that two CDs containing the information had failed to reach the NAO in London after being sent through HMRC's internal post system sparked fears they could fall into the hands of fraudsters.

The Conservatives said the emails, released by the NAO, proved the NAO had only asked the HMRC to send limited details from its database - stripping out information such as bank account numbers.

A covering letter also confirmed that a senior HMRC manager was copied in to another email rejecting the request to remove the data the NAO did not want as this would involve HMRC in additional costs. The Treasury, however, insisted that the picture revealed by the emails was fully consistent with Mr Darling's statement to Parliament.

The initial exchange of emails related to an earlier request in March by the NAO for details from the HMRC child benefit database. An email from an NAO official states: "I do not need address, bank or parent details in download - are these removable to make the file smaller?"

However, a subsequent internal HMRC email states: "I must stress we must make use of data we hold and not over burden the business by asking them to run additional data scans/filters that may incur a cost to the department." The names of senders and recipients are blacked out. However a covering letter said that a senior HMRC manager - the "process owner" for child benefit - was copied into the second email which had been sent by a junior HMRC manager.

The letter, written by NAO assistant auditor general Caroline Mawhood to HMRC acting chairman Dave Hartnett, said there was no evidence that the process owner made the decision to release the data. On that occasion the CDs arrived safely. However the NAO also released the email it sent to HMRC in October requesting that the data is sent again.

It said: "We require this data for our audit. Last time we had 100 zipped files on 2 CDs. Please could you ensure that the CDs are delivered to the NAO as safely as possible due to their content." This time the CDs failed to arrive.