UK Parking Control (UKPC) said it has investigated allegations one of its employees in Lincoln had changed timestamps on photos.
It said it had worked "to mitigate the risk of this re-occurring".
But, a wider inquiry in other parts off the UK by the British Parking Association (BPA) is under way.
The BPA said it could not comment further as they were "still in the process of our investigation".
UKPC has also been reported to Action Fraud and the complaint is currently being assessed by the National Fraud Intelligence Bureau.
Alex Shipp, who campaigns as the Parking Prankster, said: "I think this merits a full investigation, rather than just acknowledging the few cases we know about so far."
The issue came to light in Lincoln when Simone Riley-Young was issued with a parking charge notice while parked at Tritton Retail Park on 18 July.
Drivers can park there free of charge for up to two hours.
The timestamps on the photos said 07:15 and 09:27, but Ms Riley-Young said the earlier time was impossible as she was still in bed.
Another driver, Matthew Treacy, had the same problem at the car park on 23 August.
The timestamps on the photos of his car said 14:06 and 16:21, but Mr Treacy said he had four independent sources to prove his car could not have been in the car park at 14:06.
UKPC operates more than 1,400 sites nationwide and similar problems have been reported outside Lincoln, including in Newcastle-under-Lyme, Staffordshire.
Talking about the Lincoln incidents, UKPC said there had been a full investigation.
"This behaviour is completely contrary to UKPC's operating procedures and employees who do not adhere to our high professional standards or codes of conduct are subject to disciplinary proceedings up to and including dismissal," a spokesman said.
"We have put steps in place to mitigate the risk of this reoccurring."
The BBC contacted UKPC again about incidents elsewhere in the country, but the company has not responded.
A car park operator is being investigated over claims photographic evidence has been altered to unfairly impose parking charges.