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.
Falmouth said its claims were based on a subset of five "arts universities" in three league tables, but the ASA said the claim was only generated by "narrowing the pool of competitors".
The Advertising Standards Authority examined a range of claims - including the University of Leicester stating that it is in the "top 1%" in the world - and found that none of them could prove the assertions to be true.
On the basis that there are around 26,000 universities in the world, the University of Leicester said it would need to be ranked in the top 260 in order to be counted in top one per cent.
The University of West London, also a former polytechnic and ranked 79th, made a bogus claim that it was "top modern university" in the capital.
The universities said they based their claims on independent assessments provided by compilers of national and worldwide league tables.
Officials found that Falmouth University was unable to substantiate its claim to be the "UK's No 1 Arts University" after an academic complained that there were other higher-ranking universities offering creative courses.
"Our rulings send a clear message to United Kingdom universities", said Guy Parker, the chief executive of the ASA.
"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", he said.
"In the old days the top-tier universities filled up first and the other ones filled up afterwards", he said.
"We are disappointed that the ASA differs in their interpretation of the rankings from the compilers of the tables themselves".
Several of the universities said they disagreed with the ASA but would abide by their 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.
" While we are disappointed with the ruling, we have included an additional reference to the Times Higher Education in our statement". "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".
The University of West London said: "UWL has fully complied with the ASA ruling. The advertising that was the subject of investigation has been removed and will not appear in our future marketing materials".