A woman saw a UFO "20 times the size of an aeroplane" while driving home on the M2 - but no-one else spotted it.
The sighting has been revealed in the latest cache of UFO files released by the Ministry of Defence.
A Ms Wood from Rainham e-mailed the MOD on March 28, 2008, to voice her shock.
She said: "Whilst driving home on the M2 between the Rochester and Gillingham turn-off, at approximately 8.20pm this evening, I am convinced I spotted a UFO.
"At first glance I saw two white lights far ahead. Initially I thought it was an aeroplane, however after looking a little longer I suspected the lights were too large and bright.
"As I drove towards the Gillingham turn-off the object got much closer until it was right above me to my right.
"I saw a huge disc-shaped object approximately 20 times the size of an aeroplane with approximately five or six flashing lights on the underneath, in a circular shape.
"I slowed the car down enough as there was no traffic directly near me. I managed to get about five or six glances at it as it just hovered above me. It was most definitely not an aeroplane or a helicopter as we know it."
Ms Wood added she spotted another object in the sky half an hour later while she was unloading her shopping.
"It was extremely loud, low-flying and was in the shape of a triangle with three lights," she said.
The MOD wrote to her saying it "remains totally open-minded" about alien life.
But the letter added: "I can confirm we received no other reports of UFO sightings for March 28, 2008, from anywhere in the UK."
The MOD's UFO hotline existed to establish if enemy states could be invading British airspace, but closed down in 2009. Reports elsewhere in Britain included a man who said he was living with an alien in Carlisle, and a UFO taking a dog, a car and a tent.
Another man recalled seeing a UFO in July 1954 and called in to report it in January 2008.