Police holding four men arrested on suspicion of a terrorist plot to buy radioactive material have been given more time to question them.
The men are being held at Paddington Green station
Three of the men were arrested at a Brent Cross hotel on Friday and a fourth in a north London house.
The arrests followed a tip-off from a Sunday newspaper about the alleged plot by a group of businessmen.
The extension has been granted to Friday at 1330 BST. The men can be held for a total of 14 days without charge.
The information on the alleged plot was passed to police by the News Of The World.
The newspaper claimed it sent in an undercover reporter posing as a "Muslim extremist" following a tip-off that a Saudi sympathetic to "the Muslim cause" was willing
to pay £300,000 for a kilo of powerful, radioactive "red mercury".
The chemical is said to have been developed by Russian scientists for "briefcase nuclear bombs", although scientists are divided over whether any
According to the News of the World, meetings were held with men
hoping to supply the radioactive material to their Middle Eastern buyer.
'Not highly significant'
A BBC correspondent said police had not found any radioactive or bomb-making material but that searches were ongoing.
The men - who are being questioned at Paddington Green Police Station in London - were arrested on suspicion of commissioning, preparing or instigating acts of terrorism.
BBC Home Affairs correspondent Margaret Gilmore said: "The allegation is that these men, who we understand are all businessmen, were trying to buy radioactive materials.
"The suspicion, and this is what the police are questioning them about now, is that they were trying to get hold of radioactive materials to sell them on to terrorists.
"They don't believe they have arrested highly significant people, on the other hand they have possibly stopped something in the making," she added.
Gary Thompson, associate editor of the News Of The World said the story followed covert investigations by its reporter Mazher Mahmood.
"He alerted police who made the arrests. We do not know what specific details there were of any targets (for bombs)," he said.
Police marksmen and surveillance teams surrounded the hotel in Brent Cross on Friday and the arrests were made.
A Metropolitan Police spokesman said on Saturday: "Several addresses have been searched. Some searches continue."
MI5 were also involved in the operation, it has been revealed.