Like everybody, I could not believe what was happening

Sir Paul McCartney

More than 300 firefighters are thought to have perished in the collapse of the two towers.

Sir Paul said: "I witnessed the last moments of the World Trade Center twin towers.

'Heroism'

"Out of the window of the plane I could see the towers smoking and in flames and, like everybody, I could not believe what was happening.

"Through staying in New York for some days after the attack I was able to also witness the tremendous heroism that has come out of the city, including the bravery of the firefighters.

"I have great admiration for the courage those guys showed."

"I feel a connection with them because my father was a volunteer fireman in Liverpool during World War II," he said.

Ringo's association with McCartney goes back to 1962

Sir Paul is already due to play a huge benefit concert at New York's Madison Square Garden alongside Mick Jagger, Eric Clapton, The Who, Bon Jovi, James Taylor, Billy Joel - and McCartney's old Beatle bandmate Ringo Starr.

"We're doing this big benefit concert in New York to honour the rescue workers and to benefit the victims of the attacks, and I hope that the sales of this new single will help to raise more money for the firemen and their families," said Sir Paul.

The single will be released worldwide on 29 October, nine days after Sir Paul's concert.