A spokesman for the Oil Ministry said Tehran didn't need the business and had new customers lined up for the barrels.
"Exporting crude to British and French companies has been stopped," the unidentified spokesman said on the ministry Web site. "We will sell our oil to new customers."
Iran has warned several European nations they would also be cut off in retaliation for new international sanctions aimed at Iran's nuclear program. The European Union had pledged to stop importing Iranian oil starting July 1.
The British newspaper The Guardian said the move could push up the price of Brent crude, a blend of North Sea oils that is used a benchmark for crude futures.
China, India, Japan and Italy are considered the biggest customers for Iranian crude, which accounts for the lion's share of Iran's export revenues.