Thousands of people are dropping out of apprenticeships every year, after firms provided little or no training, according to a report.
The most recent official data suggests that nearly half of apprentices fail to complete their courses.
EDSK, a think tank, found that the majority of people quitting cited "poor quality", including lack of training and bad management.
The government said it is introducing reforms "to boost quality".
Under apprenticeship schemes, which receive government funding, those aged 16 years and over must get one day a week "off the job" to receive teaching and training.
EDSK found that many get less than the minimum weekly requirement while some receive nothing at all.