The UK's two biggest supermarkets have been accused of deceiving shoppers over loyalty card savings.

Consumer group Which? has accused Sainsbury's and Tesco of using "potentially dodgy tactics" to convince customers of greater Nectar and Clubcard savings than they actually are.

Which? examined 141 loyalty card prices and looked at their pricing history over the previous six months, as reported by Retail Gazette.

It showed that around a third of the member-only promotions could only be bought at the 'regular' price quoted for less than half of the time period.

The consumer group also found regular prices being changed just before the loyalty card promotion and regular prices being much higher than at other supermarkets.

Which? said that not all Tesco and Sainsbury's customers could register with the supermarkets' loyalty schemes due to eligibility rules, including age restrictions.

The consumer group has asked the Competition and Markets Authority (CMA) to look into the findings, to see whether supermarkets are raising their normal prices to mislead shoppers.

In response, a Sainsbury's spokesperson said inflation meant price rises for many products over the last six months.

And a Tesco spokesperson said its regular prices were in line with those charged by other stores at the time.