Islamic terrorists were on the trail of a nuclear bomb which they hoped to buy from the Russian mafia.
As efforts were made to negotiate its sale in Europe, a British terror suspect was asked: "Do you want something ready-made, something which will be bigger than 9/11?"
But the internet chatroom exchange between Salahuddin Amin in Pakistan and Omar Khyam in Britain in early 2004 was intercepted.
Amin was involved in discussions to buy a nuclear bomb, the court was told.
After going to Pakistan in 2001 he was approached by an al Qaida contact called Abu Munthir, someone he had met at a mosque in Luton, to contact a man about a nuclear device.
David Waters QC, prosecuting, said: "An indication as to the trust imposed in Amin and his position in the Pakistani end of the organisation is perhaps gained from the passing of information to him in relation to a radio-isotope bomb.
"Abu Munthir asked Amin to contact a man named Abu Annis on Munthir's behalf.
"Amin did so via the internet and Abu Annis said they had made contact with the Russian mafia in Belgium and from the mafia they were trying to buy this bomb.
"Amin told the police in interview that he didn't believe this could be genuine.
"In his own words, he didn't think it was likely 'that you can go and pick an atomic bomb up and use it'.
"And indeed, nothing appears to have come of this. However, it perhaps gives an indication as to Amin's position in, and his usefulness to, the organisation."
Mr Waters said that whether the possibility of acquiring and using a radio-isotope bomb were realistic or not, Amin had made an immensely important contribution to the conspiracy to cause explosions.
In a recorded interview with British police, Amin said: "He (Annis) told me that they had made contact with the Russian mafia in Belgium and from them they are trying to buy a radio-isotope bomb.
"I went back and had a word with Abu Munthir about it and he said he would get back to me ... but he never did, so there was no further development on this issue.
"I don't even know if there is such a thing as a radio-isotope bomb. These things are not easily available on the open market.
"It's not like that, that you go and pick up an atomic bomb and use it."