Children taught by staff who aren't their usual teachers make less progress in lessons, according to a joint report by the school's inspectorate Estyn and the Wales Audit office.
The report says that supply teaching is often less effective because staff don't know enough about the needs of the pupils they teach.
It added that the pace of lessons can also be slow and expectations too low.
Authors have recommend that supply teachers gain better access to the national training programmes that are available to permanently-employed teachers
The Welsh Government says it will consider the report's recommendations and respond in due course.
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Wales head coach Rob Howley admitted his side's performance 'simply wasn't good enough' against Scotland this afternoon.
It will be cloudy with outbreaks of rain at first, but most of the rain will clear by morning.
Despite leading at half time, Howley's men failed to score in the second half with Scotland racking up 20 points to seal the win.