Brian Clark is an only child who gained a brother at the age of 54. He and Stanley Praimnath met and bonded in the face of death on the 81st floor of the World Trade Centre on 11 September 2001.

Clark felt the south tower list violently and feared it would topple over. Praimnath stared spellbound at a passenger jet coming directly towards him, glimpsing the 'U' of 'United Airlines' on its tail before it buried its wing in his office door. The pair were among only four people at or above the tower's impact zone who lived.

Four years after the terrorist attacks killed nearly 3,000 people, they still count their blessings - including a close friendship forged high in the burning skyscraper.

'Your friends are people you shared experiences with,' Clark, 58, told The Observer. 'Stanley and I for sure have shared an experience, and we're still friends.'

Praimnath, 49, who has two young daughters, said: 'Had it not been for Brian Clark, there would not be a Stanley Praimnath. My wife would not have a husband today. The kids would not have their father. So this man is more than a brother.'

On the morning of 11 September the two men - whose story is told in a documentary, United by 9/11, to be shown on Sky One next weekend - had never met. Clark, an executive vice-president of Euro Brokers, worked on the 84th floor. Guyana-born Praimnath, a loan officer for the Fuji Bank, was based on the 81st. The first hijacked jet had already smashed into the north tower when Praimnath took a call from a colleague in Chicago who saw it on TV and pleaded with him to get out.

'I was looking towards the Statue of Liberty and telling her no, I'm fine. Something caught my eye: a giant airplane, with U on the tail. I said, "I have got to go. A plane is aiming for me." I dropped the phone and jumped towards my desk, which was six or seven feet away.

'I said, "Lord, I can't do this. You take over," and I went into the foetal position. I just huddled under my desk and prayed and cried.

'Just before I jumped there I saw this plane eyeball to eyeball, the biggest thing I've ever seen coming towards me. But it was happening in slow motion, giving me time. I could hear this ripping engine sound, and the bottom wing just swiped right through my office. It crash-landed and the bottom wing was stuck in my office door 20ft from where I was. Everything looked like a demolition crew ripped the entire floor apart.

'I thought, if I don't get electrocuted, the plane is gonna blow, I'm gonna die. If that don't get me, the air pressure's going to suck me out. I'm trapped under the only desk that stood firm - my Bible's on top of that.

'As I'm screaming, "Lord, send somebodyto help me," there's somebody with a flashlight, shining it all around. I'm saying to myself: this can't be real, I've gotta be dreaming, and I'm screaming, "Don't leave me here to die".'

The flashlight belonged to Clark, who had survived the impact on the the 84th floor where 61 of his colleagues perished. The Canadian, who was office fire warden, said: 'There was a boom and the room fell apart. The building swayed an intolerable distance, 6ft to 8ft, then back to vertical. I felt terrified, frozen to the spot, thinking: "My God, what's going on?"'

But Clark heard Praimnath's cries and went to help - a decision which saved both men's lives. Clark said: 'His voice was guiding my flashlight. "Come this way, come that way," and "I can't breathe". Suddenly I could see his hand, less than a yard from me. My flashlight went down to this face, these eyes, poking out of this hole in the wall and this arm stretched out. "Hallelujah, I've been saved!" said this voice.

'He said, "Do you believe in Jesus Christ?" He's quite an evangelical Christian. I stammered, "Well, I go to church every Sunday." He was trapped behind a wall. There was debris in the way. Then I realised there was a chance for him to go over the wall. So I climbed up and reached down and said, "Now you must jump." He jumped, scrambled up and he didn't quite make it. I said, "You must do this, this is the only way out".'

Clark helped Praimnath over the wall, pulling on his hand and holding his head. 'We fell in a heap, on my back, him on top of me,' Clark said. 'This stranger gave me this big kiss, and I said, "Uh, I'm Brian," and he said, "I'm Stanley, we'll be friends for ever". He said, "Will we be brothers forever?" and I said, "I don't have any brothers." He said, "Well, I'll be your brother."

During their desperate, hobbling descent of the 1,620 steps to ground level, Praimnath made an incongruous gesture which proved invaluable. 'I slipped Brian a business card. We got down and we're running. It's like a scene from Forrest Gump. We'd got three blocks - thousands of people there cheering us along, "Run, run, run!"' Later, they were separated, and Clark used the card to contact Praimnath.

The men's lives became irrevocably intertwined. They talk every couple of weeks and meet regularly. When Clark's daughter married a few weeks ago, Praimnath and his wife were guests at the top table. Praimnath, who gives talks about his experience to churches around the US, said: 'Brian is like a big brother. He calls me to check I'm OK more often than my own brothers. He's a heck of a guy. If I ask him for anything he will do what he can, and I do the same for him.'

· 'United by 9/11' is on Sky One at 9pm on Sunday 11 September