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 airports, turn it around and crash it into one of the four terminals. (read more)