A BMJ review of studies covering nearly 9,000 patients found that giving patients advice and encouragement in surgery was just as effective as referral to exercise classes.
Authors led by Dr Gillian Orrow at the University of Cambridge said the intervention could be used in a similar way to schemes for smoking cessation and alcohol reduction.
Researchers examined data from 15 studies from Western countries including the UK, USA and New Zealand. These explored trials of physical activity promotion based in primary care, including GP practices.
Approaches included motivational interviews with patients about their activity levels, booklets about the benefits of exercise alongside a letter from the GP, personal activity plans and behavioural counselling.
In all studies reviewed, patients, aged 17-92, were followed up after at least 12 months to check for improvement in self-reported activity levels.
Patients were 42% more likely to see an improvement in physical activity 12 months after taking part in a primary care trial.
Researchers found that 12 adults would need to take part for one to sedentary adult to reach 150 minutes of moderate intensity physical activity per week as recommended by the UK's CMOs.
In comparison, 50-120 smokers must receive cessation advice for one to successfully quit.
Only one study compared results of primary care-led schemes with referral for exercise schemes such as fitness classes or health walks. But this found no difference in effect.
Authors wrote: 'Five years after NICE recommended that exercise referral interventions should not be commissioned in primary care outside of well designed research studies, we found insufficient evidence to challenge this guidance.'
Researchers said it was likely primary care-based schemes would be cost effective. They added that research has suggested that population-wide achievement of CMO-advised activity levels could cut mortality by one fifth.
Study authors said the nature of self-reported outcomes means the results should be met with caution.
Nevertheless, they concluded: 'Our findings raise the question of whether brief single contact interventions delivered in primary care can be as effective as more intensive approaches, including exercise referral schemes.
'Brief interventions have shown effectiveness in the promotion of smoking cessation and alcohol reduction, and further research is warranted to assess their effectiveness for physical activity promotion.'