The report analysed 26 government and local authority websites, including those run by the Department for Work and Pensions, UK Online and the NHS, to assess their ability to provide relevant information and services in response to typical inquiries on benefits, health, housing and consumer complaints.
Researchers found information and services on government websites were "far from reliable".
They also said the ability of the flagship UK Online website to act as a "signpost" to other e-government services was disappointing.
David Harker, chief executive of Citizens Advice, said, "It is vital that public service bodies think carefully about how they design services and how they provide information via the internet."
A spokeswoman for the Office of the E-Envoy, which is responsible for co-ordinating public sector efforts to deliver services electronically, said the government was committed to building online services around the needs of customers.
"We regularly seek feedback from stakeholders on how best to do this," she said.