The findings follow on-site inspections of almost 6,000 cars – both company and private – being used for business.
“If non-essential service and repair costs are not covered by the allowance, our evidence shows that cars are neglected,” said Simon Hill, of fleet consultants Total Motion.
“We estimate that around a third of drivers surveyed were in this position.”
The problem stems from drivers being tempted to take non-maintenance contracts provided by brokers when they are not entitled to a company car or have chosen to opt out and take cash.
The difference between a contract with full maintenance cover and one without is typically around £60 to £80 per month.
By opting for a non-maintenance contract, an employee can easily upgrade from a Ford Mondeo to a BMW 3 Series.
A fully-maintained Mondeo 2.0-litre diesel estate from one internet broker costs £491 a month on a 36-month/60,000-mile contract.
The same company offers a BMW 320d Touring for £474 a month on a non-maintained contract.
“But when a driver discovers that new tyres on a prestige car cost £250 each and brake discs can add another few hundred pounds, they find they can’t afford the work,” said Mr Hill.
“It is the brokers that are creating this problem. If a driver has £300 a month, he should be sold a fully maintained contract, but then he is told he can afford a prestige brand, but what he is not told is the additional maintenance costs he faces.”
Mark Sherry, of internet broker Car Partner Fleet Solutions, which advertises all its vehicles with non-maintained contracts, said the industry was becoming aware of the issue.
“This is definitely a problem,” he said “We find that employees of small companies generally take non-maintained contracts. They are more price sensitive.”
Mr Sherry says he now advises anyone driving more than 20,000 miles a year to take a contract with maintenance cover included.
However, the decision lies with the employee. “And they often don’t think about the maintenance costs,” he said.
ACFO chairman Julie Jenner said more stringent checks or a move back to company cars were the only answers.
“ACFO has always recommended that fleet operators ensure that service records and MoT certificates, where required, are provided to the company – and on a regular basis,” she said.
“In reality, though, I am not sure that this happens as it can be an administration burden where companies have a large number of opt-outs.
“Of course, this problem doesn’t arise when company cars are taken.”