The Home Office is not confirming or denying whether any British workers have been involved.
But officials say they are investigating reports which suggest at least one Briton was taken when gunmen stormed a compound belonging to a Lebanese construction company in Nigeria.
ITV News reporter Sejal Karia talks from outside the Foreign Office:
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The prince laid a floral wreath at one of a group of metal crosses near Wellington Arch in central London.
The group, which also owns Premier Inn, said on Wednesday that it will pursue a ‘demerger of Costa’.
The showers will become more frequent as the day goes on, giving some heavy downpours in places during the afternoon.