The advertising watchdog has banned "misleading" claims by six United Kingdom universities about their rankings, student satisfaction and graduate prospects.
The majority of complaints that were upheld about the universities pertained to misleading or unsubstantiated claims that they were "number one" for certain degree subjects.
Teesside University had claimed on its website and on Twitter that it was the "top university in England for long-term graduate prospects" based on the government's 2016 longitudinal outcomes data.
An ASA ruling said: "The ASA considered that, in the absence of further qualification, the average consumer would interpret the claim "top 1 per cent world university" to mean that the University of Leicester had been objectively ranked within the top 1 per cent of all universities in the world".
At the time, the university's head of corporate communications Charles Heymann said he welcomed the clarification over how universities can present global rankings but said the ASA needed to investigate "every single other United Kingdom university which claims it is in the top 1% in the world, rather than waiting for individual complaints to be made".
ASA chief executive Guy Parker said: "Our rulings send a clear message to United Kingdom universities". If you're making claims about your national or global ranking, student satisfaction or graduate prospects, make sure you practice what you teach: play by the advertising rules, in particular by backing up your claims with good evidence. We were not provided with sufficient evidence to support the figure of 26,000 universities worldwide on which the claim in the ad was based.
The watchdog said that it felt so strongly about the issue of potentially misleading students - who are facing leaving university with huge loans that many will struggle to repay - that it is issuing new guidance to universities to make sure they abide by United Kingdom ad laws.
A Universities UK spokesman said: "Universities take their responsibilities to use data appropriately in advertising and marketing extremely seriously".
"We are disappointed that the ASA differs in their interpretation of the rankings from the compilers of the tables themselves. While we disagree with the ASA on their assessment methodology, we will abide by their ruling".
Falmouth University said it was "disappointed" with the ruling.
The University of Strathclyde said its physics department was "rated number one in the United Kingdom for research in the REF [research excellence framework] 2014", but the ASA said it should have made clear that the basis of this claim was an analysis of the REF results by Times Higher Education rather than the REF itself. "As well as sending this message, we're also issuing new guidance to help universities get their ads right so students can be confident they'll get what they pay for".
Guy Parker, chief executive of the ASA, said the rulings "send a clear message to United Kingdom universities".