The seizure of 15 British Navy personnel by Iran is a "very serious situation", Tony Blair said yesterday as he publicly entered the diplomatic crisis for the first time.
The Prime Minister appealed for a swift end to the "unjustified and wrong" detention of the sailors and Marines who were snatched on Friday.
He warned Tehran that it was a "fundamental" issue for the UK and insisted they had not strayed into Iranian waters.
"I have not been commenting up to now because I want to get it resolved in as easy and diplomatic a way as possible, because it is the welfare of the people that have been taken by the Iranian government that is most important," he said. "But this is a very serious situation."
Speaking from an EU meeting in Berlin, he said: "There is no doubt at all that these people were taken from a boat in Iraqi waters. It is simply not true that they went into Iranian territorial waters, and I hope the Iranian government understands how fundamental an issue this is for us.
"We have certainly sent the message back to them very clearly indeed. They should not be under any doubt at all about how seriously we regard this act, which is unjustified and wrong."
Mr Blair said he hoped the issue could be resolved diplomatically in the next few days, adding: "But the quicker it is resolved, the easier it will be for all of us."
Mr Blair's intervention came as the UK continued to press the Iranian authorities to reveal where the 14 men and one woman were being held.
The British ambassador to Tehran met foreign ministry officials in the capital yesterday in a fresh bid to secure their release.
But Foreign Office Minister Lord Triesman, who held an hour of "frank" talks with the Iranian ambassador on Saturday, admitted it was still unclear where they were. "We do not know where they are, I wish we did," he said amid reports the sailors and Marines, including one woman, had been taken to the capital.
Lord Triesman said he had been assured the group was not being held hostage for political reasons - as the UK joined other UN Security Council members in imposing tougher sanctions on Iran over its refusal to suspend its nuclear programme.
The troops, from the Type 22 frigate HMS Cornwall, were carrying out a routine search of a large cargo dhow which they suspected of smuggling.
A Tehran-based news agency claimed the seamen have admitted illegally sailing into Iranian waters and that satellite tracking systems on the British boats proved they were inside Iranian territory.
The capture and detention of the British service personnel risks escalating an already fraught relationship between Iran and the West.
Tehran last night announced it was partially suspending co-operation with the UN nuclear watchdog agency, the Vienna-based IAEA, because of the latest UN Security sanctions imposed on it.