The US navy came within seconds of firing at the Iranian speedboats in the Strait of Hormuz on January 6 after hearing threats that the boats were attacking and were about to explode.

Senior navy officials have admitted that the source of the threats, picked up in international waters, was a mystery.

And now the US navy's journal, Navy Times, has claimed that the threats, which were broadcast last week by the Pentagon, are thought to have come from an infamous radio prankster.

It said the Filipino Monkey, who could be more than one person, listens to ship-to-ship radio traffic and then interrupts, usually with abusive insults.

Rick Hoffman, a retired captain, told the paper: "For 25 years, there's been this mythical guy out there who, hour after hour, shouts obscenities and threats. He used to go all night long. The guy is crazy.

"Could it have been a spurious transmission? Absolutely."

An unnamed civilian mariner told the Navy Times: "They come on and say Filipino Monkey in a strange voice. You're standing watch on bridge and all of a sudden it comes over the radio. It's been a joke out there for years."

Last week, the Iranians and the US issued different video versions of what took place.

On the Pentagon's version, a strange voice, in English, can be heard saying "I am coming to you. You will explode in a few minutes." The voice sounds different from one heard earlier in the recording and there is no background noise that would usually be picked up from a speedboat radio.

In the Iranian version, there is no hint of aggressive behaviour.

The Pentagon said it recorded the film and the sound separately and then edited them together to give a "better idea of what is happening".

But Commander Lydia Robertson, a navy spokeswoman, admitted: "We don't know for sure where they [the threats] came from. It could have been a shore station."

The US lodged a formal complaint with Iran over the incident, and the president, George Bush, warned Tehran of "serious consequences" unless it stopped such aggression.

During the 20-minute incident, five Iranian patrol boats swarmed around three US warships and came within 200 metres, puttingthe ships on alert.

The US navy said its gunners came within seconds of firing on the speedboats.