The UK financial watchdog is facing four legal challenges against its £9.1bn compensation scheme for victims of the motor finance scandal.

The Financial Conduct Authority (FCA) said that it will defend the scheme “robustly” as the “fastest, simplest route for consumers and the most efficient way for firms to put things right”.

The FCA confirmed the Guardian’s report of a legal challenge from the consumer group Consumer Voice, which claims that the scheme massively short-changes victims. It is represented by Courmacs Legal.

The watchdog is also facing challenges from the lenders Volkswagen Financial Services, Mercedes-Benz Financial Services and Crédit Agricole Auto Finance.

The FCA said none of the claims received were expressly in the name of any individual consumers.

“We will defend the scheme robustly as lawful and the best way to resolve such a widespread, long running and complex issue,” the FCA said. “These legal challenges create fresh uncertainty for millions of consumers and for the second largest consumer credit market.”

The FCA is due to hand aggrieved borrowers £830 on average for each mis-sold loan.

The FCA said that it was “engaging at pace” with lenders and consumer groups to understand the views of all sides as it looked at next steps for the scheme, including “contingency planning”.

The legal actions dash the regulator’s hopes of drawing a line under the motor finance scandal, in which drivers were overcharged for loans as a result of commission payments between lenders and car dealers between 2007 and 2024.

The challenges could mean taking the FCA to the upper tribunal, where a judge would be asked to review the merits of the long-awaited compensation programme. That could end up delaying payouts to drivers, which were expected to begin as early as this summer.

The FCA said: “We welcome the broad support for the scheme and the commitment from most lenders to implement it.

“The final scheme is fair to consumers and proportionate for firms. [Lenders] have taken a pragmatic approach recognising that introducing a scheme on this scale promptly has required us to make judgments to simplify in a reasonable and lawful way some complex legal and operational issues. Alternative approaches would be slower and much more costly for firms.”

The FCA issued the final terms of the £9.1bn compensation programme in March. About £7.5bn will be paid out to borrowers, while the remaining £1.6bn will cover administrative costs for banks and specialist lenders.

That is a fraction of the up to £44bn that some analysts were suggesting banks could face prior to last summer’s supreme court ruling.