The firm is attempting to revive its fortunes in its home market after losing share to rivals but has suffered a series of setbacks this year, including the discovery across Europe of horsemeat in products labelled as beef.
Tesco was one of several firms forced to withdraw some goods and apologise to customers.
"We are extremely disappointed to discover a pork loin product probably came from a Dutch farm, not a British farm," a Tesco spokesman said.
"When we specify that we want British pork, we expect to be supplied with British pork. We have spoken with our supplier to make clear that this mistake is unacceptable."
The pork issue emerged after pork purchased in a Tesco store in Salford, north west England, by a reporter working for BBC Radio 4, was tested in a German laboratory using Stable Isotope Ratio Analysis.
The BBC said the British Pig Executive (BPEX) carried out the test for it to establish the effectiveness of measures intended to ensure that pork sold under the "Red Tractor" logo is from Britain.
It said test results showed there was less than a 1 percent chance that the pork came from Britain, and may have come from the Netherlands.
The discovery is embarrassing for Tesco because since the horsemeat scandal it has vowed to restore customer confidence in the provenance of its food.
Tesco said further testing on more pork products confirmed the country of origin was correct in all cases.
"We've recently trialled this new isotope testing and we are talking to BPEX about how we can develop this alongside our existing tests, to bring even more rigour to our food testing programme," the spokesman added.
Earlier this month Britain's advertising watchdog ruled Tesco misled consumers with a press campaign on its response to the horsemeat scandal by implying issues with meat standards existed across the whole food industry.
And in August the firm was fined 300,000 pounds ($476,000) by a British court for misleading consumers over the pricing of strawberries, prompting a wave of negative publicity.
In June Tesco, the world's No. 3 retailer, posted a drop in quarterly underlying sales in its main British market, resuming a trend seen for most of the past three years and raising doubts about its 1 billion pounds turnaround plan.
The company has been investing heavily in more staff, new food ranges, revamped stores and lower prices.
Shares in Tesco were up 0.6 percent at 374.3 pence at 0907 GMT, valuing the business at 30.1 billion pounds.
