Zurich university hospital has suspended a high-profile skin cancer trial, following a series of medical and procedural irregularities.
An internal investigation reported that patients might have been treated with substances that had not been cleared for human use.
The project, which aims to produce an effective vaccination against melanoma tumours, has also been criticised for charging fees to some participants and making unsubstantiated claims on its internet site as to the trial's likely success rate.
The hospital's dermatology department attracted global attention in 1998, when it published initial results from a pilot project. Out of 16 patients with previously untreatable melanomas, six were said to have responded to the vaccination.
The apparent breakthrough was reported in internationally renowned magazines, including "Nature Medicine" and "The Scientist" and led to a larger joint study with other European universities, involving 240 patients.
But the questions raised over the running of this study have now forced the Zurich hospital to suspend the research, pending possible external investigations.
Swissmedic, the national medical watchdog, has told swissinfo that it will be examining the internal report before deciding if further measures are necessary.
In a statement released to the media, the hospital insisted that no patients had been put at risk by the irregularities.
The statement admitted, however, that several patients had been asked to pay up to SFr20,000 to take part in the trial, contrary to the hospital's own rules.
The recruitment of volunteers for the project over the internet has also come in for criticism, particularly due to the use of potentially misleading statistics derived from the pilot study.
In an interview with Swiss television, hospital director Christiane Roth insisted that the figures had been correct at the time of their initial publication but said she could not go into details about possible subsequent falsification.
The hospital statement also gives no details as to the possible use of unauthorised substances, although dermatology department chairman Günter Burg did deny the allegation in a weekend newspaper interview.
Burg told the "SonntagsZeitung" that the cantonal ethics commission had allowed the trial team to use the unnamed substances up until 2003.
However the commission has since pointed out that its jurisdiction in such matters ended in 2001, after which Swissmedic was given sole responsibility for authorising the use of otherwise banned materials.
Swissinfo sought further clarification, but was told that the hospital would not be making any further comments until the close of any possible investigations.
swissinfo, Mark Ledsom in Zurich
Researchers at Zurich's university hospital are hoping to develop an effective immunisation against melanomas - the most aggressive forms of skin cancer.
Switzerland suffers from a proportionately high incidence of melanomas - with around 1,500 Swiss developing the dark tumours each year.
Following a critical internal investigation, the Zurich trial has now been suspended indefinitely.