(Adds Sainsbury's comment, paragraph 9)
By Kate Kelland and Mark Potter
LONDON, Dec 7 (Reuters) - Britain's major supermarkets cleared their shelves of all Irish pork products on Sunday because of concerns they may contain toxic dioxins.
The product withdrawal followed advice from Britain's Food Standards Agency, which warned consumers not to eat any Irish or Northern Irish pork products after tests revealed possible contamination with potentially cancer-causing dioxins.
"As a precautionary measure ... we will be immediately withdrawing from sale any products that contain pork sourced from the Irish Republic or Northern Ireland pending further guidance," Asda, which is owned by U.S. retail giant Wal-Mart
"We can trace all our meat products back to the source they come from and we're currently investigating the situation with our suppliers across all our products."
A spokesman for Britain's largest supermarket Tesco said it had removed the relevant products from the shelves of its Irish stores and was reviewing its supply chain.
"Customer safety is our number one concern and if any more action is required we will take it," the spokesman said.
Other supermarket chains said they were unaffected.
Waitrose said in a statement: "All Waitrose fresh pork is 100 percent British, sourced from farms in East Anglia and southern England."
J Sainsbury said it did not use any Irish pork in its fresh pork, bacon or sausage ranges and added: "We do not believe it is contained in any other food product. But we will investigate further and if any product is found to contain Irish pork we will take immediate action to remove it from sale."
WM Morrison said its pork was "100 percent British" and declined to comment further.
The Irish government ordered on Saturday a recall of all domestic pork products from shops, restaurants and food processing plants because of contamination with dioxin -- which in some forms and concentrations, and with long exposure, can cause cancer and other health problems.
Asda said any customers wishing to return products would be given an alternative or a refund.
Industry body the British Retail Consortium said retailers were working closely with the FSA and "as instructed by them" had withdrawn the "small proportion of fresh Irish pork products they sell".