Hezbollah militantReuters File Photo

Tension between Iran and the United Kingdom escalated after the UK's intelligence sources said that Iran-backed terrorist groups could launch an attack on Britain.

It is believed that Iran has funded terrorist cells that are linked to Hezbollah, a Lebanese militant group, all across Europe including the UK as counter-measures to the conflict in the Persian Gulf, British security service (MI5) and British foreign intelligence service (MI6) officials told The Telegraph.

"Iran has Hezbollah operatives in a position to carry out a terrorist attack in the event of a conflict. That is the nature of the domestic threat Iran poses to the UK," the source said. Hezbollah is involved in the Syrian conflict.

Iran-UK has been locked with tension after Iranian Revolutionary Guards seized British oil tanker 'Stena Impero' in the Strait of Hormuz on Friday. The tanker had 23 crew members of Indian, Russian, Latvian and Filipino nationals abroad.

British Foreign Secretary Jeremy Hunt urged Iran on Saturday to reverse its "illegal" seizure of the British tanker. He said the situation "raises very serious questions" about the security of British and international shipping in the Strait of Hormuz.

Hunt also revealed said that Iran considers the seizure as "tit-for-tat situation" following the detention of a 'Grace 1' Iranian tanker in Gibraltar earlier this month based on suspicion that Iran violated EU sanctions and was carrying oil to Syria.

The Grace 1 seizure was called as legal detainment in Gibraltarian waters "totally within the law" according to Hunt. However, Iran described the incident as "piracy".

International maritime dispute

Iranian Director-General of Ports and Maritime Organisation (PMO) Allahmorad Afifipour said on Sunday, "All 23 crew members aboard the ship are safe and in good health in Bandar Abbas port."

Recording of a radio exchange between Iranian vessel and Royal Marine's 'HMS Montrose' frigate indicated that the vessel was approached by Iranian Revolutionary Guard who said that the Stena Impero needed an inspection for "security reasons."

The audio revealed that Iranian vessel told the frigate, thought to be Stena Impero, to change its course, saying, "If you obey you will be safe."

The patrolling Naval officer responds to the Iranian vessel and said "under international law, your passage must not be impaired, intruded, obstructed or hampered,"

It is reported that after the Iranian vessel said that they are conducting a security-related inspection, the ship was boarded and seized by Iranian officials.

London has issued a warning to its ships to avoid the Strait of Hormuz passage.

The increasing tensions have raised concerns among top UK officials who have sought emergency strategy to seek the release of Stena Impero. "Our priority continues to be to find a way to de-escalate the situation," Hunt said.

Prime Minister Theresa May will announce the next steps to be taken in response to the crisis in an emergency cabinet meeting on Monday.