The RMT rail union has threatened to strike after the London Underground was hit by its second Tube derailment in less than 48 hours.

Seven people were injured when a Northern Line train carrying 200 passengers derailed in Camden.

Bob Crow, general secretary of the RMT, said: "There is this whole web of contracts and we don't believe the proper safety system is in place.

"If the London transport commissioner does not suspend these contracts then the mayor should do it.

"If he doesn't do it, then the government should. And if they don't I shall be going to my executive committee and recommending that we ballot to take strike action."

Ken Livingstone, the London mayor, said that the derailments raised grave concerns. "One obvious question will be whether, as we feared, the PPP [public private partnership] has increased the risk of incidents of this type," he said.

"Londoners and visitors to the city must be able to be sure they are safe on the Tube."