Taxpayers may face penalties if they do not file online returns
Concerns about data security have been raised after it emerged celebrities, Royals and MPs are blocked from submitting income tax returns online.
HM Revenue and Customs (HMRC) admitted "high profile" individuals must submit forms by post because they are judged to require extra protection.
But critics said equal treatment should apply to all 3m self-assessment users.
Mark Wallace of the Taxpayers' Alliance said: "This is a completely unacceptable double standard."
HMRC came under fire after it emerged in 2007 that the department had lost discs containing personal details of 25 million people.
For each tax year everyone who submits a self-assessment form after October, will have to do so online or face penalties.
"MPs should have to face the same problems with tax returns as their voters and the ordinary taxpayer," Mr Wallace added.
"It is crucial that these systems are only opened to the public when they are completely secure."
The special arrangements for MPs, Royals and celebrities were uncovered by Tory MP Andrew Robathan after he asked the government why HMRC had informed him by post that he could not file his return online.
In the House of Commons, Treasury minister Jane Kennedy told him: "There are categories of individual for whom security is a higher priority.
"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."
But an HMRC spokeswoman said it was "technical issues" which prevented higher security individuals filing online.
The online system could not access their records due to the way they were stored on HMRC's main database, she added.
She 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."