First, Asda said that from today it was cutting 1p a litre off petrol and 2p a litre off diesel.
Asda said motorists would pay no more than 128.7p a litre for petrol and no more than 135.7p a litre for diesel.
Then Sainsbury's said that, also from tomorrow, it was "dropping petrol by 1p per litre and diesel by up to 2p per litre".
RAC technical director David Bizley said: "It really is starting to look like Christmas. Not only have we persuaded the Chancellor to scrap January's 3p-a-litre fuel duty increase, but now supermarkets are making festive travel more affordable.
"An RAC poll found that three-quarters (77%) of people plan to drive an average of 260 miles to visit friends or family this Christmas. So this is good news for them."