Sainsbury's and Tesco loyalty card prices not as good as seem, says Which?

Sainsbury's and Tesco said all prices have been going up due to inflation.

Consumer rights group Which? tracked pricing history in stores for six months .

It found 29% of member-only promotions were at their so-called regular price for less than half of the six-month period.

Both supermarkets said Which ? had failed to take inflation into account.