HM Revenue and Customs was last night accused of taking a cavalier attitude to the protection of data on ordinary taxpayers after it emerged that MPs, royals and celebrities were banned from filling in their tax returns online for security reasons.

HMRC faced accusations of encouraging a two-tier system after thousands of Britons which are regarded as "high profile" are being asked to send in their applications by post while the general public – around three million users – are required to provide their details on the internet-based system. Consumer groups questioned why the same level of security was not being offered to all users after a minister admitted: "There are categories of individual for whom security is a higher priority."

The disclosure follows the highly damaging "datagate" fiasco last November in which child benefit information including names, addresses, National Insurance numbers and the bank details of 25 million people were lost in the post. The news will fuel suspicions that the HMRC's security systems are not secure.

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The restrictions were uncovered after the Conservative MP Andrew Robathan asked questions in Parliament after receiving a letter from HMRC. He said: "I received a letter recently telling me that 31 January is my final deadline for a tax return, and encouraging me to file my tax return online, saying that that was the best way for me to do so. Given our discussions on the efficiency of HMRC recently, how come I have also been sent a letter from HMRC saying that I cannot file online?"

In response, Jane Kennedy, a Treasury minister, said: "[It is] not just Members of Parliament – there are several categories of people in that position – and HMRC does not have the facilities for them to file online."

Last night the HMRC insisted that it was "technical issues" which were preventing the "higher security" individuals filing online because they are held under "separate arrangements".

A spokesperson said: "A tiny minority of individuals' records, including MPs, have extra security measures in place over and above the very high standards of confidentiality with which HMRC treats all taxpayers' data. The separate arrangements for dealing with these records mean that they are currently unable to use the online service. HMRC is continuing to look at ways to extend the online filing service to this group."

But Mark Wallace, of the TaxPayers' Alliance, said: "This double-standard is unacceptable. If the online system is not secure enough for MPs, why should ordinary taxpayers put up with it?"