Brendan Vaughan, the British man thought to have been killed in Nigeria by an Islamic extremist group, was "likely to have been murdered in cold blood", the Foreign Secretary said.
William Hague added that the UK had "worked closely" with Nigeria over the last few weeks, and expressed his thanks to the Nigerian authorities.
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The president-elect also warned US companies they would face "retribution" for setting up their business operations abroad.
Penalties of £100 or a court fine of up to £1,000 can now be handed to anyone caught lighting up in a private vehicle with someone under 18.