"The Foreign Secretary spoke this morning to his opposite number Kharrazi and Kharrazi said he would look into it personally," the Foreign Office said.
Tehran claims the British servicemen were a thousand yards inside Iranian territory.
"Even if they have strayed into Iranian water technically by a few yards, in one sense, so what?" said Conservative Party foreign policy spokesman Gary Streeter.
The Iranian Arabic-language satellite channel al-Alam has shown footage of the eight crew members sitting in a room, looking very somber, and they've also shown footage of equipment seized from the boats, which is radio equipment and several weapons, reporter Ramita Navai in Tehran told CBS Radio News.
"According to the Iranian news channels the British sailors
said that they did make a mistake. This can happen at some points on the Shatt-al-Arab waterway," said Navai. "There's only a mile distance between the Iran-Iraq borders."
According to state-run television in Iran, the eight crewman are to be prosecuted. But the British government has not been told so formally.
"We are seeking clarification of the reports that they are going to be prosecuted," said a Foreign Office spokesman, on customary condition of anonymity. "We are in constant dialogue with Tehran."
Streeter wants the government to take a hard line.
"There's no justification for holding these men. They've got to be released," he said.
Iran said Monday it had seized the boats and crew for entering Iranian territorial waters.
The waterway, Iraq's main link with the Persian Gulf that divides Iran and Iraq, has long been a source of tension between the neighbors. The 1980-88 Iran-Iraq war broke out after then-Iraqi leader Saddam Hussein claimed the entire waterway.
Diplomatic relations between Britain and Iran have been unpredictable for years, and have been strained recently over Iran's nuclear program, reports CBS News Correspondent Steve Holt.
Britain has pursued a policy of constructive engagement with the clerical regime and last year, with France and Germany, persuaded Tehran to cooperate with international nuclear inspectors.
But relations are again strained, after London helped draft a resolution at last week's meeting of the International Atomic Energy Agency, for failing to meet its commitments.
British diplomats sought to play down a possible link between the resolution and the arrest of the eight crewmen, however, and suggested the arrests had been made by an opportunistic local military commander.
"I hope this doesn't get caught up with the bigger negotiations going on at the moment about the Iranian nuclear capability," said Streeter, the shadow foreign secretary. "The pressure that's being applied on them by the international community has got nothing to do with it whatsoever."
With the June 30 handover of sovereignty to an Iraqi government approaching, some analysts are saying that this is simply Iran asserting its control over the Shatt-al-Arab waterway, which has been a cause of many conflicts between Iran and Iraq, said Navai. "It was actually a catalyst of the eight-year Iran-Iraq war."
The British Defense Ministry said the personnel from the Royal Navy training team based in southern Iraq was detained while delivering a boat from Umm Qasr to Basra, Iraq, to the new Iraqi Riverine Patrol Service.
The ministry said their boats were unarmed but the crew were carrying their personal weapons.
The Royal Navy has been training Iraqi personnel in coastal defense for several weeks on the Shatt al-Arab waterway, and it is possible that the vessels out of radio contact were taking part in such an exercise.