Car park management company Parkingeye was brought in March to reduce over-stays and unwanted parking from people not using the stadium or the leisure centre.
However, fans using the club shop have said the signs are “too small” and are “catching people out”, leading to fines.
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It is also claimed that the 20-minute grace period is not being adhered to by the parking company, which was introduced by landowner The Firoka Group of Companies.
Parkingeye insists the car park uses “prominent and highly visible” signs and describes the 20-minute grace period as “reasonable”.
Visitors using the leisure centre or supporters going to the ticket office or club shop can extend the grace period by up to four hours if vehicles are registered via a QR code.
Season ticket holder Denise Moss used the car park ahead of last month’s play-off final at Wembley to buy bucket hats and merchandise for her daughter’s 18th birthday.
She said she registered her vehicle inside the shop but this appeared to have not worked after receiving a parking charge notice four days after using the car park.
Ms Moss, who also had a blue badge, admitted she was “shocked” to receive a parking charge notice for being 19 seconds over the 20-minute grace period.
She said: “I did everything right.
“I parked and scanned the code but I am still being penalised and £100 is a lot of money for someone just trying to support their local team at Wembley.”
Ryan Vickers claimed he received a notice after using the car park for 17 minutes on May 22 and is now currently appealing the penalty.
Mr Vickers described the parking process as “ridiculous” and a “total money-making scam”.
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One fan, who asked to remain anonymous, shared a letter with the Oxford Mail showing a £100 parking charge notice for being in the car park for 11 minutes while picking up an Oxford United kit for their one-year-old daughter.
They have since appealed and have brought the total down to £20 but are continuing to fight to decision and are now waiting for a further decision.
A representative from the Oxford United Fans Page said: “Oxford United Fans should be celebrating the fact we have reached the Championship for the first time since 1999, racing to the club shop to get the merchandise for the new season and to show the club support.
“They should not be worried about receiving fines for parking within the 20-minute grace period.
“It’s not just a problem for Oxford United, but anyone using the leisure facilities based at the Kassam too.
“It wouldn’t be such a big problem, but it appears that the 20-minute grace period is being ignored.”
A Parkingeye spokesperson said: “The car park at Kassam Stadium and Leisure Centre features prominent and highly visible signs throughout providing information on how to use the car park responsibly.
“This includes guidance on how all motorists must enter their vehicle registration into the terminal or via a QR code at one of the onsite premises, in order to obtain a four-hour permit.
“Motorists parking on match days do not need to register unless they are visiting the car park four hours before kick-off or four hours after the match.
“To ensure users of the car park have time to read the signs and decide whether to stay or leave there is a reasonable consideration period which is currently 20 minutes.
“This is not free parking and both the club and Parkingeye would like to highlight once again that all visitors to the car park must register their vehicle using the terminal or the QR code at the stadium or leisure centre.”
Oxford United have not provided a comment with the club not being responsible for parking restrictions at the site.
The Oxford Mail has approached Firoka Group of Companies for comment.
Oxford United have played at the Kassam Stadium since 2001 but the club is currently in the planning application process to build a new 16,000-seat stadium in Kidlington.
Source: Fans hit out at ‘money-making scam’ at Kassam Stadium car park