As many as a third of all payments fraud cases will not be covered under new reimbursement regulations due to be implemented next week, raising concerns over how the rules will incentivise behavioural changes of fraudsters and fraud detection specialists.

Under the new Authorised Push Payment (APP) scams rules introduced by the Payment Services Regulator (PSR), which are due to come into force on 7 October, there will be a £100 excess on fraud claims, combined with a £85,000 cap on reimbursement payments.

That means that claims under £100 would not be eligible for any reimbursement, while reimbursement claims over £100 would be reduced by £100. For example, a successful £110 fraud claim would result in a £10 reimbursement, while a claim for £500k would lead to a £85k payment.

Scams under £100 account for 32% of all APP scams, according to the PSR’s data, while scams over the value of £85,000 account for around 1 in 500 cases.

The regulator said the limits would minimise financial harm to consumers while ensuring that caution is maintained in the system by payment services providers (PSPs). It conceded that the move could encourage a behavioural shift among fraudsters towards more scams near the £100 threshold, while discouraging PSPs from bothering to investigate fraud under the threshold.

“We recognise the risk that a fixed maximum claim excess would focus PSPs’ resources on the detection and prevention of higher-value fraud, while weakening incentives on PSPs to tackle low-value fraud, which could result in fraud migrating to below the level of the excess,” the PSR said.

“A maximum claim excess of £100 is an effective way of encouraging customer caution. While other excess options would better combat moral hazard for smaller and larger payments, when considered along with other factors, a £100 fixed excess remains the best option.”

The PSR added that claims for transactions under £100 could still be considered valid if the total claim value exceeded £100, for example if it involved multiple transactions of £50 where it could be proved that they were connected to the same fraudster.

The PSR previously proposed capping the total reimbursement at £415,000 but has since scaled this back, a move which consumer groups have warned could cause acute harm to the hundreds of consumers each year who are victims of six-figure scams.

Rocio Concha, Which? director of policy and advocacy, said a decision to lower the reimbursement cap would “reduce the incentives for banks and payments firms to take fraud prevention seriously”.

“The regulator has shamefully sidelined scam victims, despite the evidence showing that this decision could have a negative financial and psychological impact on them,” she added.

On Thursday, fintech giant Revolut called for social media firms to be liable to reimburse victims of scams that originated on their platforms.