Rupert Cushny, 68, spoke out ahead of the 20th anniversary of the atrocities after investigators last year named Mussaed Ahmed al-Jarrah as a mystery ‘third man’ assisting the terrorists.
Al-Jarrah worked at the Saudi embassy in Washington DC.
He is said to have instructed two people to help Nawaf al-Hazmi and Khalid al-Mihdhar settle in the US when they arrived in the country ahead of the attacks.
Al-Hazmi and al-Mindhar took part in the hijacking of the American Airlines plane that flew into the Pentagon on September 11, 2001, killing 125 people.
In all, more than 3,000 people were killed in the atrocities which also saw two planes hit the Twin Towers in New York and a fourth crash in Pennsylvania.
Among 67 British victims was 47-year-old Gavin Cushny, an IT expert from the Isle of Lewis working for Cantor Fitzgerald on the 104th floor of the North Tower, the first building to be hit.
In all, 15 of the 19 hijackers were from Saudi Arabia and the families of victims have filed a lawsuit accusing the government in Riyadh of being complicit in the attacks.
Gavin’s brother Rupert, an author from Warlingham, Surrey, said: “The Saudi involvement has always been extraordinarily fishy, but I think it’s extremely important that any wrongdoing is exposed to the wider world, specifically by Saudi Arabia.
"With this name coming out I believe it shows the Saudis did assist the hijackers.”
Lawyers have alleged the desert kingdom funded al-Qaeda – the terror network that carried out the attacks – to stop home-grown fundamentalists turning against the ruling royal family.
The terror mastermind, Osama bin Laden, was from a prominent Saudi family before setting up training camps in Afghanistan.
Rupert said: "If you look at it, it’s Saudi Arabia’s brutality and horrific regime that gave birth to Osama bin Laden. It is a very repressive regime, still living in the Dark Ages in terms of civil liberties and suppression of minorities.
"They just happen to be incredibly wealthy and have lots of oil, so we bend over backwards for them.”
Al-Jarrah – whose name was accidently left unredacted in court papers by an FBI official – is believed to have tasked two men, Fahad al-Thumairy and Omar al-Bayoumi, to help the hijackers.
At the time al-Thumairy was a Saudi Islamic Affairs official and radical cleric who was the imam of the King Fahd Mosque in Los Angeles, and al-Bayoumi was a suspected Saudi government agent.
Investigators believe when the two Pentagon hijackers arrived in California, al-Bayoumi found them a flat, lent them money and set up a bank account for them.
The Saudi government denies any involvement in the 9/11 attacks and the 9/11 Commission report found “no evidence that the Saudi government as an institution or senior Saudi officials individually funded” the atrocities.
But the commission did note “the likelihood” that Saudi-government sponsored charities did.
Brett Eagleson, a spokesman for the 9/11 familes whose father was killed in the attacks, said: “This shows there is a complete government cover-up of the Saudi involvement. It demonstrates there was a hierarchy of command coming from the Saudi Embassy to the Ministry of Islamic Affairs [in Los Angeles] to the hijackers’.
Gavin, a former student at St Andrews University, had just got engaged to his American fiancé Susan Brady when he was killed.
He died trying to escape down the stairwell of the North Tower and his body was found in the rubble two months later surrounded by 12 dead firemen.
Rupert recalled watching the drama unfold on television, unaware that his younger brother was inside one of the buildings under attack.
“I’d seen him at our father’s funeral six months earlier, but I hadn’t realised where he would be working,” Rupert said.
"It was only when one of his best university friends phoned my elder brother that I found out.
“For a long period of time I assumed I’d watched my brother commit suicide without knowing it [by jumping from the building] or gone up to the top floor and not been rescued by a helicopter.
“However, Gavin was an extraordinarily brave man with a phenomenal desire for life and in early November his body was found intact except below his knees with another victim and 12 dead firemen. He’d managed to break out, but sadly not far enough.”
Last week a friend of Gavin’s from St Andrews told of a chilling conversation she had with him after another terrorist attack on the Twin Towers seven years earlier.
Joyce Lewis told the Daily Record: “While we were discussing it Gavin gazed up at the Twin Towers, then turned to me and said ‘can you believe that some people working inside there actually walked down more than 80 stairs?’
But in a horrible twist of fate, Gavin went on to work in the Twin Towers and was the only employee at Cantor Fitzgerald to survive the blast from the planes – only to die in the stairwell trying to get out.
Last week US president Biden ordered a review and declassification of documents relating to the 9/11 attacks, following a campaign by relatives of victims seeking evidence of Saudi Arabian involvement.