Three in four universities are breaching consumer law by failing to provide prospective students with vital information on their websites such as information about tuition fees and the contact hours they will have with staff, according to a report.
Which? said it found three institutions, Canterbury Christ Church University, Glasgow Caledonian University and the University of Huddersfield, were consistently adopting unlawful practice by failing to provide more than 30% of the information required when it investigated them in September.
The Competition and Markets Authority (CMA) published advice in March setting out how consumer law applies to the higher education sector, including measures to ensure information is available to students so they can compare courses and make an informed choice.
A spokesman for the University of Huddersfield said it took "great exception" to the findings.
"The allegation that it is not compliant with a number of the CMA guidelines is completely false," he said. "The University is confident that it is fully compliant with consumer protection legislation, in line with the CMA's advice.
"We strongly refute the allegations made."
Which? said nearly two thirds (64%) of institutions failed to provide students with up-to-date information on course fees, and four in five (80%) did not state or provide clarity on any extra fees students may have to pay to complete the course.
Overall it found 76% of the 50 universities it looked at are breaching consumer law by failing to provide prospective students with vital information.
It pointed out that with undergraduate higher education fees averaging £8,900 a year in 2015/16 and students' overall average debt totalling more than £45,000, whether to go to university is the most significant financial decision many young people will take.
While no institution consistently provided good practice across all areas, Leeds Trinity University and University of Greenwich demonstrated good practice across a number of areas, the consumer watchdog said. This included details on the number of contact hours students can expect, the workload and assessments for their course and fees they will be expected to pay.
Which? said it had found the majority of students (75%) use a university's website or their prospectus to make their first and insurance choices with UCAS, and therefore some may not have been able to make a fully informed decision about their course choice.
The investigation involved analysis of the information available to prospective students for 2016/17 psychology courses across a third (50) of UK higher education institutions' websites.
The findings will be submitted to the CMA.
Which? executive director Richard Lloyd said: "Students deserve to know at least the basics of what they can expect from a course before signing up, so it's disappointing to find that a large number of universities are still breaching consumer law.
"It's encouraging to see some providers demonstrating good practice, but we now need all universities to make better information easily available and accessible for prospective students."
Professor Les Ebdon, director of Fair Access to Higher Education, said: "It is important that students have clear information about their course - including the fees payable and the financial support to which they may be entitled.
"OFFA (Office for Fair Access) requires institutions to commit to publishing clear and accessible information for prospective and current students about tuition fees. When we approved access agreements for 2016/17, we contacted institutions to remind them of this requirement.
"This study by Which? - which looked at university websites during a short period of time for one course - suggests that some institutions have fallen short of these standards. We will be writing again to institutions to make our expectations clear."
Nicola Dandridge, chief executive of Universities UK, said: "It is important that prospective students are given clear and relevant information which enables them to make informed choices about courses.
"The university sector will continue to work with the Competition and Markets Authority (CMA) to ensure that the sector is complying with its statutory requirements relating to consumer protection law.
"In relation to consumer protection law, it is important to recognise that the relationship that exists between a student and their university is a distinctive relationship to do with learning and teaching, rather than a standard consumer contract. Universities UK will continue to support universities in how best to fulfil their responsibilities in this developing area."