Supermarket loyalty schemes at Tesco and Sainsbury’s are ‘not all they’ve cracked up to be’ claims a leading consumer group.

Which? says it fears standard prices are at times being inflated to make the ensuing discount offered by holding a loyalty card look like a better deal than it really is.

Both supermarkets - which share around 43% of the grocery market – have disputed the claims.

They argue that Which? has failed to take into account price rises over the last six months which have instead been caused by record rises in inflation.

Both retailers also stress that that all the rules attached to their loyalty schemes and pricing structures fit with guidance set out by Trading Standards departments.

In its investigation Which? says it tracked more than 140 Clubcard and Nectar card prices for six months, to see whether the ‘regular price’ offered had been the standard price for a reasonable amount of time.

Researchers claim that in around a third of cases the member-only deals were at their so called regular price for less than half of the six month period.

Among the prices to have raised concern was a Sainsbury’s advertised jar of Nescafe instant coffee costing £6 with a Nectar card or £8.10 without. However Which? claims the regular price had been £6 at stores until it was increased to £8.10 two days before the Nectar price was launched.

The same jar, says the consumer group, was also more expensive than at other supermarkets including Asda where the jar was £7, at Morrisons and Waitrose where it was £6.

At Tesco Which? says it found Heinz Salad Cream with a Clubcard price of £3.50 and a ‘regular’ price of £3.90. However it says the regular price had been £2.99 for several weeks before it was increased to £3.90, 22 days before the Clubcard promotion was introduced.

Which? says it has shared its findings with the Competition and Markets Authority (CMA) – asking them to look at whether supermarkets could be increasing regular prices to make loyalty scheme members appreciate they are getting a good price for an item.

The consumer group has also raised concerns that the growth of member-only pricing structures in supermarkets risks excluding vulnerable groups from the cheapest prices such as young parents and carers, schoolchildren buying lunch or those in temporary accommodation. This is because sign-up schemes often carry age and address-based restrictions and require an element of access to technology.

Sue Davies, Which? Head of Food Policy, said: “As member-only pricing continues to grow, the sector, its pricing practices and who is eligible for membership needs to be properly scrutinised so that all shoppers – including society’s most vulnerable – can benefit and no one is misled into buying things they wouldn’t have usually bought or which isn’t quite the deal they believe it to be.”