Four men were still being questioned late this week after a News of the World investigation unveiled a suspected plot to buy radioactive material for a dirty bomb.

The four were arrested by New Scotland Yard officers on Saturday after a sting operation by NoWinvestigations editor Mazher Mahmood.

Mahmood had been working on the story for a month after receiving a tipoff that a London merchant banker was looking to buy radioactive material.

Associate editor Gary Thompson said: “Normally we would contact police late on in an investigation but because this was so serious, we decided to contact them at an early stage and worked very closely with them.”