Ken Livingstone has ordered a full audit of work carried out by Capita, the company running London's congestion charge. .
The move follows threats of court action against hundreds of motorists over fines they had already paid. .
In a serious breakdown in enforcement of the £5-a-day charging scheme, Capita has admitted it failed to process nearly 1,600 cheques from drivers who had paid penalty charges after being caught on camera inside the zone. .
Half of the drivers received letters demanding sums of up to £120 for late payment. They were told that unless they paid, they risked action from bailiffs and court proceedings. The threats prompted 450 people to pay again - money which Capita has been forced to refund. .
The error, blamed on the failure of a printer at Capita's Coventry call centre, has infuriated the mayor's Transport for London authority, which has sent a special investigation squad to check that the company is up to the job. .
A TfL spokesman said the authority took the mistake "very seriously". He continued: "The error has caused inconvenience and distress. All the individuals concerned will receive a letter of apology. .
"We intend to conduct a full audit of all the financial procedures carried out by Capita in connection with the scheme." .
Capita is understood to have begun disciplinary action against staff responsible for the setback, which was caused when a printer used to stamp bar codes on the backs of cheques broke down. The bar codes are used by Capita to link the cheques to case references, with the details of drivers, vehicles and camera footage. .
The move comes a week after the mayor revealed that just seven cars have been impounded since the charging scheme began in February. TfL had made much of its intention to crack down on refuseniks by towing away and crushing the cars of persistent offenders. .
There has also been criticism of the number of fines which have been successfully challenged.
