Thousands of staff working for HM Revenue and Customs have given IT a poor rating in an internal survey on the merged departments.
Liberal Democrat shadow chancellor Vincent Cable said the survey had given a "massive thumbs-down from staff to the way in which HMRC and the government in general is managing IT".
He added, "It is difficult to see how the Treasury can deliver on its revenue objectives if the majority of HMRC staff have little confidence in the effectiveness of their own management and the technology they have been asked to employ."
But Richard Bacon, an MP on the Public Accounts Committee, praised HMRC for publishing the results of the survey, if only internally.
The survey was carried out in May and June this year, nearly a year after Capgemini took over IT systems and services at Inland Revenue from EDS. Customs and Excise contracted out most of its main IT systems to Fujitsu.
More than 7,000 HMRC staff took part in the survey, which asked whether they agreed that IT developments in the past 12 months have improved the service they give to customers. Only 23% agreed, 29% disagreed and a further 13% strongly disagreed.
Responding to the survey, HMRC executives said they needed to "get out more into the business and talk to people face-to-face, explaining the bigger picture and listening to views".