THE mastermind of the 9/11 attacks warned that Al-Qaeda has hidden a nuclear bomb in Europe which will unleash a "nuclear hellstorm" if Osama bin Laden is captured, leaked files revealed today.

The terror group also planned to make a 9/11 style attack on London's Heathrow airport by crashing a hijacked airliner into one of the terminals, the files showed.

Khalid Sheikh Mohammed told Guantanamo Bay interrogators the terror group would detonate the nuclear device if the Al-Qaeda chief was captured or killed, according to the classified files released by the WikiLeaks website.

Sheikh Mohammed, the self-professed mastermind of the September 11, 2001 attacks on the United States, has been held at Guantanamo since 2006 and is to be tried in a military court at the US naval base on Cuba over the attacks.

His nuclear threat was revealed in Britain's Daily Telegraph newspaper, one of several media outlets which have published the classified assessments of detainees at Guantanamo.

The German weekly Der Spiegel, also citing WikiLeaks, said that Sheikh Mohammed had told his interrogators he had set up two cells for the purpose of attacking Heathrow in 2002.

The aim was to seize control of an airliner shortly after take-off from Heathrow, one of the world's busiest aiports, turn it around and crash it into one of the four terminals.

Sheikh Mohammed said one cell had been formed with the aim of taking flying lessons in Kenya, while the other had been tasked with recruiting participants.

He said the plot had been discussed several times at the highest level of Al-Qaeda. One component had involved the infiltration of ground staff at the airport, according to Der Spiegel.

Another attack given the green light in late 2001 would have targeted "the tallest buildings in California" with hijacked airliners, Der Spiegel reported.

The attackers would have gained access to the airliner cockpits by setting off small bombs hidden in their shoes, it said.

Sheikh Mohammed, captured in 2003 in Pakistan, also claims to have personally beheaded US journalist Daniel Pearl in 2002 with his "blessed right hand" and to have helped in the 1993 World Trade Center bombing that killed six people.

Der Spiegel noted that his "confessions" should have be treated with caution as they could have been extracted through torture. Sheikh Mohammed is known to have undergone the method known as "waterboarding."

Former US president George W. Bush claimed in his memoirs published last year that using the interrogation technique - which simulates drowning - helped prevent planned attacks on Heathrow and London's Canary Wharf business district.

He also told the London Times newspaper in November that it was "damn right" that he had authorised use of the method on Sheikh Mohammed.