They have picked up so many bad habits, they are an accident risk.
The findings sparked calls for drivers to be retested regularly.
Road safety charity Brake is calling for compulsory tests but other experts say drivers should be encouraged to take voluntary refresher training.
The investigation by What Car? magazine looked at 20 drivers aged 19 to 70 who volunteered to retake their practical L-test.
Nine failed, suggesting 45 per cent of British licence holders would do likewise. Eight of the volunteer drivers committed errors that were so serious, an examiner would have failed them on the spot.
Brake’s deputy chief executive Julie Townsend said: “This adds to the weight of the argument that drivers ought to be retested on a regular basis throughout their driving careers – particularly when they get older.”
Paul Atkinson, of What Car? Driving School, said: “There is a strong case for systematic additional driver training every five years.”
From October, L-drivers must also follow a memorised route in the test.