Mistakes made when ordering are the main cause of the problem, according to e-commerce group Epyx, which has found that almost 30,000 wrongly-specified vehicles annually are delivered to fleets.
Ken Trinder, head of business development at Epyx, said: 'If you are lucky, the customer will accept the wrong colour or the wrong alloy wheels, providing you make a goodwill gesture, but the customer can refuse to accept the vehicle. You then have an unwanted car that needs to be disposed of.
There is also the cost of providing a hire car to the customer while his or her correct re-order is processed by the factory and that will probably take a minimum of four weeks. The total can run into thousands of pounds.'
The UK's largest leasing company, Lloyds TSB autolease, has introduced a series of additional checks to ensure that coding on orders is correct in a bid to eliminate the problem.
Operations director David Kershaw said: 'With online systems, it is technically possible to order the wrong vehicle. We use the telephone to communicate with customers, confirming that the right model and specification is ordered.
'We keep confirming before the vehicle is ordered to reduce the problem. However, we still have on average one mistake a month, which is usually down to human error.'
Epyx's research showed that the majority of mistakes were caused by human error when ordering, with the order keyed incorrectly or faxes completed with the wrong information.
Trinder added: 'The level of error uncovered by the research we have undertaken among contract hire companies and dealers is astonishing.
'They range from simple mistakes like specifying the wrong colour to the much more practical problem of ordering a four-door saloon instead of a five-door hatchback.'
Mark Connor, purchasing manager at Lex Vehicle Leasing, said the onus was on the supplying dealer to ensure its vehicles were correct.
He said: 'We manage all our dealers through a service level agreement and it is up to the dealer to ensure that the car comes in correct. We get the odd one which is wrong, but these go straight back to the dealer.'