The study, which involved 583 GPs, was published in April's issue of the British Journal of General Practice.
It involved GPs being sent three vignettes describing hypothetical sick leave scenarios. These were back pain, depression and a combination of the two.
The research showed that fit notes will mean more people are assessed as fit for work.
GPs using the old Med 3 found that 76% of back pain sufferers unfit for work compared with 19% using the fit note.
The option of ticking ‘fit for some work' meant 70% could have returned to a certain level of employment.
But the study found doctors were uncertain about when and under what circumstances a patient was expected to return to work if the GP assessed them as ‘you may be fit for some work' on the new form.
GPs are more likely to provide written advice on the fit note as compared with the old Med 3.
The study showed that 54% provided written advice on the new form compared with 12% on the old.
When asked for suggested improvements to the fit note, doctors said there should be a clear time period for which the fitness for work assessment applies.
They also believed the space for diagnoses was insufficient.
The fit note will be introduced from 6 April.