IT is the X Files style mystery that has left UFO spotters baffled.

A silent, spinning black triangle has been caught on film flying above Southampton three times in the past month - over Millbrook, Shirley and the Western Docks.

A new video - the clearest so far - was captured on a mobile phone by a passenger travelling in a car under the Millbrook flyover.

The footage - shot on Sunday, June 1 - has caused a buzz among UFO enthusiasts from across the country who have described it as the best they have seen for some time.

An investigation was even launched by Southampton Airport after a worried member of the public alerted bosses.

An airport spokeswoman said: "We received an inquiry about a UFO on Tuesday, they reported seeing a black triangle flying near Millbrook.

"We have used our web-track technology, which tracks the flight path of aircraft, but it found nothing unusual."

Southampton's flying triangle was first filmed floating high above houses in Shirley on Monday, May 5, by company director Will Campbell, 28.

He said: "We could barely see it with the naked eye, so I got my video camera out to zoom in to get a clearer picture of the object.

"At first it was slowly rising, but then seemed to stabilise at an altitude and move away into the distance. We could see it for about 15 minutes, before it eventually got too far away."

Click to watch UFO 1 video or UFO 2 video or UFO 3 video

Mr Campbell was confident the object was not a kite as there was "absolutely" no wind that day. He added: "Its large arc across the sky also means that it was not tethered to the ground in the way an advertising blimp would be."

Steve Gerrard, founder of the now defunct Southampton UFO Group, said flying black triangles were not a new phenomenon, but had always remained a mystery.

"I have no idea what it is, that footage shot near the Millbrook flyover looks almost too good to be true. When you get a video that is so clear and sharp you have to treat it with some scepticism," he said.

$64,000 question "However, it's very interesting as it looks too high to be a kite and it looks too crisp to be a balloon. Whether it is terrestrial or extra terrestrial is the $64,000 question."

Mr Gerrard said he had received 23 reports of mysterious objects flying over the city since the start of the year.

He added: "If this was seen by three people then it must have been seen by more. I would urge anyone who has seen anything like this to report it as we need to corroborate all the evidence."

A spokesman for the National Air Traffic Service, based in Swanwick, said it passed on all UFO sightings to the Ministry of Defence (MoD).

The MoD was unavailable for comment yesterday, but it's the department's policy to only investigate sightings that pose a specific threat to the country.

Totton College psychology lecturer Craig Roberts, who co-ordinated Britain's first ever ufology course, said previous flying black triangle sightings had been explained by stealth military aircraft and tricks of the camera.

"There has always been in years gone by a lot of black triangles spotted, but it has been years since I have seen anything as vivid as this," he said.

Other popular theories include Chinese lanterns, weather balloons, microlight and model aircraft, kites or even a secret military project.

The British UFO Research Organisation (BUFORA) said detailed information such as the make of camera, specific location, date, time and direction of travel was needed to further analyse the footage.

"There have certainly been many reports over the years of black triangles, particularly from the Derbyshire area," BUFORA chief investigator Heather Dixon said.