Iran has accused the UK of playing a "dangerous game" and demanded the British Navy release the supertanker seized off the coast of Gibraltar last week.
It comes a day after Gibraltar Police arrested the captain of the Iranian Grace 1 supertanker, thought to be delivering oil to Syria, which would be against EU sanctions.
On Friday, Gibraltar's chief minister Fabian Picardo confirmed 2.1 million barrels of light crude oil were found on the 1,085 foot vessel.
Calling for its release, a spokesman for Iran's foreign ministry told the state news agency IRNA: "This is a dangerous game and has consequences.
"The legal pretexts for the capture are not valid... the release of the tanker is in all countries' interest."
British Royal Marines stormed the tanker off the coast of Gibraltar on suspicion it was breaking EU sanctions.
The arrest was made after authorities seized documents and electronic devices from the vessel.
The altercation came a day after Iran spoke out against Britain due to the interception of a supertanker believed to be breaching EU sanctions by carrying a shipment of crude oil to Syria.
Defence Secretary Penny Mordaunt said the government was "concerned" by Iran's actions and urged Tehran to "de-escalate the situation".
Conservative leadership hopeful Jeremy Hunt said Iran's attempted blockade of a British mercantile ship was a "concerning development".
He added: "I'm very proud of the Royal Navy and the role they played in keeping British asset, British shipping safe.
"We are continuing to monitor the situation very, very carefully."