Tesco, which owns c-store chain One Stop, said the trial would give consumers more variety and choice at the sites. A national roll-out would depend on the success of the trial stores and customer feedback, it added.
"We are keen to find out if this range better meets the needs of customers shopping on our forecourts," said a Tesco spokesman. "One Stop will offer customers a high-quality, wide range of convenience goods, including newspaper and magazines, confectionery, household items and gifts."
The move could give Tesco the opportunity to increase profit from its forecourt shops, according to Association of Convenience Stores public affairs director Shane Brennan.
"It could be a margin grab. In the petrol station shops Tesco has a range of products requiring, say, four members of staff and so it has an operating overhead. But it has to provide the same prices as over the road in the superstore," he said. "By bringing in the One Stop brand they can inflate the prices and therefore the margins. People are generally unaware that Tesco owns One Stop."
The fascias were swapped over a couple of months ago, according to staff at the trial stores.
Tesco confirmed it had introduced One Stop products in the two stores and that it had retained the original staff.
However, the ACS claimed the move called One Stop's declared independence from its parent company into question.
"Tesco says One Stop operates independently from it, but if it can swap between the two in a trial surely that suggests they aren't operationally independent," said Brennan.
Tesco has 440 forecourt stores attached to its superstores and is understood to have costed a full roll-out of the One Stop fascia.
Accounts filed at Companies House this week revealed One Stop's sales rose 10.2% to £499m in the year to 28 February.