Technology analysts thought the devices could pose a serious challenge to the market leaders.

BBC: Amazon launches new Kindle Fire tablets in UK

Still, there are some highly dangerous and violent prisoners who pose a serious challenge to prison discipline and safety.

NEWYORKER: Hellhole

But those opposed to an elected Upper House, said to include several senior members of the Cabinet, believe they would pose a serious challenge to the supremacy of the Commons.

BBC: Irvine backs appointed Lords

But "it's certainly a very serious thing" that could pose a long-term environmental challenge.

CNN: Oil spill could be disaster for animals, experts say