The number of people passing the driving theory test has dropped by nearly a third, from 70 per cent to just 50 per cent, over the last six years.
The reduction in the number of successful attempts is the result of changes to the test, aimed at ensuring candidates have a proper understanding of the theory around safe road driving.
This is part of the Driving and Vehicle Standards Agency's (DVSA) policy of 'constant review', which sees the test modified every so often to make it more relevant for real life on the roads.
The formal theory test was introduced in 1996. Before then, drivers were simply asked questions on the Highway Code by their examiner at the end of their practical test.
However, a number of changes have been made over the years, in an attempt to improve the quality and standard of testing. In 2007, the number of multiple-choice questions was increased from 35 to 50, and in 2009 a case study element was added, with five questions relating to a specific motoring scenario.
2012 saw a ban on new test questions being published online or in practice papers, and the question pool was refreshed in 2013, meaning that drivers have access to very few real test questions when studying.
The test's pass rate peaked in August 2008, when 70.6 per cent of those taking it walked out with a certificate. In the period between April and June this year, that was down to 50.7 per cent.
Commenting on the fall in pass rates, a Department for Transport report said: "This fall in pass rate is a continuation of a reduced pass rate following the changes to questions in January 2013.
"It may also be associated with the withdrawal of voiceovers and translators and foreign language tests from April 2014."
Conversely, however, the pass rate for the practical element of the driving test is on the rise, with 47.1 per cent of people passing, compared to 44.2 per cent in 2007-08.
A spokesman for the DVSA said: "The theory test requires candidates to demonstrate they have a good knowledge of the rules of the road and the theory behind safe driving," the BBC reports.
"We keep the test under constant review to ensure it remains as effective as possible.
"In January 2012 we stopped publishing the theory test questions to make sure that candidates understand the theory behind safe driving, rather than simply learning answers by rote."
The theory test currently costs £31, but planned price reductions will see that drop to £23 by October 2015.