These loans incentivised car dealers to offer higher interest rates in return for a higher commission, without sufficiently informing borrowers.

Consumer association Which! estimated that millions of drivers would become eligible for compensation if the UK’s highest judicial authority sides with borrowers.

The government, however, sought to intervene in the case amid concerns over the economic fallout.

“Such a substantial sum could limit banks’ ability and willingness to lend and provide credit at a time when the economic outlook remains uncertain,” said AJ Bell investment director Russ Mould.

“This may be why the government is seeking to intervene,” he added.

Marcus Johnson is one of the claimants whose case is under consideration by the Supreme Court.

He took out a loan in 2017 when he bought a Suzuki Swift from a car dealer in Cardiff, Wales for £6,500 ($8,400) — unaware that interest paid on the loan would fund a commission of over £1,600.

The Court of Appeal in October ruled in favour of Johnson, ordering South African lender FirstRand Bank to refund the commission plus interest — sparking panic across the finance sector.

Now, the Supreme Court will hear his case on April 1, along with another similar case against FirstRand and one against British bank Close Brothers.

If the court sides with borrowers after the three-day hearing, it will set a precedent for similar cases across the country, potentially triggering billions in compensation.

“In each of these three linked appeals, the claimants were financially unsophisticated consumers on relatively low incomes,” the Supreme Court said in its case summary.

It has rejected the government’s attempt to intervene.

£44 billion

In preparation for the ruling, British banks have set aside considerable sums, including Lloyds Bank, which has earmarked nearly £1.2 billion.

Which! estimated it could cost banks up to £16 billion, while other analysts expect the sums to be higher, with those at HSBC suggesting it could hit £44 billion.

That “would put it on a similar scale to the payment protection insurance (PPI) scandal, where the major UK high street lenders reportedly paid out some £45 billion to £50 billion between them,” said Mould.

The Financial Conduct Authority banned undisclosed commissions in 2021 and launched a separate investigation into the practice early last year.

The financial watchdog plans to wait for the Supreme Court ruling before deciding whether to start a programme for automatic compensation.

“Even if the Supreme Court upholds the Court of Appeal judgement, it could act to limit and reduce any compensation payments, and that could well represent the best-case scenario for the lenders,” Mould added.