Employees found doing so will breach the Revenue's computer misuse policy, and could face a ban on promotion, financial penalties or, in extreme cases, sacking or prosecution.
The warning came as the chairman of the Inland Revenue, Nick Montagu, said staff had been "heroic" in processing nearly 4 million claims for new tax credits "despite the computer systems not working as we expected".
Hundreds of thousands of claimants had not received their money on time. The Inland Revenue said much of this was due to its "slow" tax credit computer systems.
Writing in the Revenue's internal newspaper Revenews, Montagu said chancellor Gordon Brown and paymaster-general Dawn Primarolo had also expressed their appreciation of staff.
But in the same issue Jim Harra, director of the Tax Credit Office, warned staff who have made claims that they should not try and view their records on the system. More than 50,000 Revenue officials have access to tax credit information.
He said a special unit dealt with claims by staff for tax credits. "Like the main tax credits helpline, the staff helpline has sometimes been unable to answer all the calls they have received," he said, apologising for the poor service.
But he warned any staff who are tempted to check their record on the system, "You will not be able access it and your attempt will breach the department's computer misuse policy."
Computer Weekly revealed earlier this year that the Revenue had issued a new computer misuse policy, which said staff faced stiff penalties if they breached the code.
Harra's warning came in response to a tax official who had written to the Revenue newsletter to complain that staff with problems claiming tax credits had been unable to get through to the helpline.
"To be greeted with an answering machine and then told to phone back later, and then to be cut off is not, I think, good customer service. Imagine if I did that to a taxpayer," said the official.
A £150m system to support new tax credits, built by the Revenue's main IT supplier EDS, went live in April. But a "big-bang" approach coupled with failing computer-to-computer links led to a backlog of hundreds of thousands of unprocessed claims.
Staff said the systems are now working normally but there is still a backlog of about 90,000 claims and some calls to the Revenue's helpline are going unanswered.