Concerns about plagiarism in a 2013 law module should have triggered a formal process, says the Quality Assurance Agency for Higher Education.
Instead academics reduced students' marks for "poor academic performance".
The university says this was more appropriate as the problem was poor referencing rather than plagiarism.
The QAA review found that academics at the university initially raised concerns about some five essays, or 10% of the number submitted, which had the fault of "quoting directly from a source and failing to insert quotation marks around the quoted passages".
The university later referred to these sections as having been "lifted".
However, the university's own inquiry found that the students, then at the start of their first year, had not yet been subject to rigorous training on avoiding plagiarism and had not committed any deliberate action to deceive.
So instead of moving to its formal plagiarism procedure it "determined upon a mark reduction for over-reliance on 'one or two articles' and a subsequent interview with the students to explain", says the review.
The university told the QAA team that having first entertained the possibility that the students in question had plagiarised, it then discounted the possibility.
"It did not judge it to be plagiarism as defined in the university's regulations."
It was "deemed not sufficiently serious in nature for it to be recorded on the students' records".
However, the watchdog concludes that the reference to the students' lack of training and the view that it was not done to deceive appear to be mitigations of plagiarism rather than rejections of the possibility of plagiarised work.
"The evidence appears to show that the level of 'suspicion' involved in the work in question met the criteria that should have triggered formal procedure."
The review finds that the university had substituted its investigation and imposition of penalties for due process, "and thus was in breach of university regulations on academic misconduct".
Any reforms to the university's regulations should avoid allowing course teams to make decisions in isolation which would risk "a less consistent approach to its management of academic misconduct", recommends the review.
In a statement, the university said it took the matter of academic misconduct "extremely seriously".
"In noting the review team's various recommendations, we welcome the conclusion that our procedures for academic misconduct are fit for purpose and conform to the expectations of the UK Quality Code; and that we are taking positive steps to educate and help our students to avoid plagiarism.
"The university has completed a full review of its procedures, and will shortly be submitting an action plan to QAA detailing how we are addressing the recommendations."
The University of Buckingham breached its own regulations in an investigation into possible plagiarism, an independent inquiry has found.