British oil tanker Stena Impero, believed to have been captured by Iran, at an unknown location. Intercepted radio communications between a British warship and Iran's Islamic Revolutionary Guard Corps revealed that the British navy tried but failed to prevent the seizure of its tanker. File photo: Stena Bulk via AP.

DUBAI - Intercepted radio communications between a British warship and Iran's Islamic Revolutionary Guard Corps revealed that the British navy tried but failed to prevent the seizure of a British tanker in the Strait of Hormuz, fueling a wave of recriminations in London on Sunday over who is to blame for the incident.

In the tape, obtained by the shipping consultancy Dryad Global and posted on its website, the voice of an Iranian with the Revolutionary Guard is heard ordering the Stena Impero tanker to divert course toward Iran. 

"If you obey you will be safe," the man said.

A British naval officer interrupts, telling the Stena Impero that it has the right to proceed through the waterway. 

"Under international law your passage must not be impeded, obstructed or hampered," he said.

The British officer then addresses the Iranian: "Please confirm that you are not intending to violate international law by unlawfully attempting to board the MV Stena."

He was speaking from the British frigate HMS Montrose, one of two warships sent to the Persian Gulf to protect British shipping from Iranian threats to seize a British tanker in retaliation for Britain's detention of an Iranian tanker in the Mediterranean earlier this month.

In London, Britain's defense minister defended the actions of his country's navy, pushing aside a charge by a former admiral that the British-flagged oil tanker should have been better protected ahead of its interception on Friday.

Tobias Ellwood, the defense secretary, said the priority now must be for Britain to "de-escalate tensions" with Tehran after Iranian forces confiscated the tanker in the Strait of Hormuz on Friday.

"Our first and most important responsibility is to make sure that we get a solution to the issue to do with the current ship, make sure other British-flagged ships are safe to operate in these waters and then look at the wider picture," he said on Sunday.

Asked by Sky News whether Britain had taken its "eye off the ball," and failing to defend ships in crucial waterways, Ellwood replied: "No, not at all."

Shortly after the audio conversation took place, Iranian commandos wearing ski masks swarmed the deck of the tanker on a rope descending from a helicopter, as Iranian fastboats closed in by sea, according to a video of the incident posted by Iranian media.

The Stena Impero has since been taken to the Iranian port of Bandar Abbas along with its 23 crew, most of them Indian nationals. All of the crew members are safe, Iranian media quoted Iranian officials as saying on Saturday.

The circumstances of the tanker's seizure have come as a reminder of how difficult it is going to be for the international community to secure the safety of shipping in the Persian Gulf and the narrow Hormuz Strait that controls access to it. The United States has also sent naval reinforcements to the area, and is trying to encourage other allies to join it in the formation of a coalition to protect commercial shipping.

A fifth of the world's oil passes through the narrow, crowded waterway, but many countries, such as China, rely on the route for a far greater percentage of their energy needs.

On Sunday, Iranian Foreign Minister Mohammed Javad Zarif warned Britain not to escalate the situation. In a posting on his Twitter account, he accused the U.S. national security adviser, John Bolton, of seeking to drag Britain "into a quagmire."

"Only prudence and foresight can thwart such ploys," he tweeted.

The standoff comes as the British government and lawmakers are distracted with the finale of the leadership contest to succeed outgoing Prime Minister Theresa May, a race between the current Foreign Secretary Jeremy Hunt and the former foreign secretary Boris Johnson.

According to the polls, Johnson is expected to win and enter 10 Downing Street on Wednesday afternoon - with the ongoing crisis with Iran waiting on his desk.

In a column in the Observer newspaper, Allan West, a former first sea lord and chief of Naval staff, wrote that the British government's warning that British-flagged vessels avoid the passage through the Persian Gulf was "not good enough."

"We should have enacted control of shipping procedures, directing ships to assemble in safe areas and then taken them through in convoy," he said.

"Whoever the next prime minister is," West wrote, "he is going to face a major international crisis as soon as he is in post. It cannot be ignored because of Brexit.

"Some powerful groups in Israel, Saudi Arabia and the United States want war and think a precision strike against key parts of Iran's military capability would lead to regime change. They are wrong. It would lead to an open-ended war with catastrophic consequences across the region and the globe," he added.

Britain's seizure of an Iranian tanker, the Grace 1, near Gibraltar had prompted Iranian threats to seize a British tanker in retaliation. The Iranian tanker was seized on the grounds that it was transporting oil to Syria in violation of European Union sanctions targeting Syria's government. But Iran's retaliation has thrust Britain into the heart of the ongoing dispute between Washington and Tehran over the United States' withdrawal from the Iran nuclear deal and subsequent imposition of harsh new sanctions on Iran's economy.

West's critique that the country was distracted was unintentionally supported by the top story in the Telegraph newspaper with the headline: "Boris Johnson's biggest dilemma if he enters No10 - what to do about Carrie Symonds?" A story about whether the likely new prime minister will cohabitate with his girlfriend at the official residence. Johnson is in the middle of a divorce with his second wife and dating Symonds, a former Conservative Party communications official.

The Washington Post