Worcester City Council is planning to take legal action against car park operator Flowbird over delayed customer repayments.
More than 200,000 duplicate payments have been taken from customers' accounts, with around £400,000 incorrectly received that has yet to be returned.
The council's director of finance, Shane Flynn, said: ‘This is a frustrating situation. In some cases, people have been left hundreds of pounds out of pocket – this is completely unacceptable and, in our opinion, a gross breach of the contract we have with Flowbird and for that reason we have issued a contractual breach notice.
‘The council is not absolving itself of responsibility in trying to get this issue sorted and we are doing all we can to keep customers updated with accurate information on the situation, but, despite our best efforts, Flowbird has offered little information over the past week.'