The British SAS is unable to mount a rescue mission to save the English aid volunteer being held hostage by Isil because "we don't know where he is", the country's Foreign Secretary admitted last night.
Philip Hammond said that the family of Alan Henning, who Isil is threatening to murder, has been told there are "limitations to our abilities" although it would be a "different story" if he could be located.
David Cameron has promised to hunt down the terror group that has killed David Haines, another British aid worker, and is now threatening the life of Mr Henning.
Last night the prospect of air strikes against Isil faced complications. Senior Conservatives fear that Ed Miliband could oppose action, as he did when military involvement in Syria was voted on last year.
Tory whips are working to convince 30 rebellious Conservative MPs to support air strikes ahead of Mr Cameron's appearance at the UN General Assembly next week.
It also emerged last night that official House of Commons analysis suggests strikes against Isil in Syria without the consent of Bashar Al Asaad, the country's president, could be illegal.
At the weekend it emerged that Mr Haines had become the first Briton to be killed by a man believed to be the terrorist known as 'Jihadi John', who speaks with a British accent and has previously beheaded two US journalists.
In the footage, released on Saturday night, the terrorists threaten to murder a second British man, Mr Henning, 47, an aid convoy volunteer from Manchester who was captured in Syria late last year.
It emerged yesterday that he was kidnapped within half an hour of entering the country after he unwittingly became involved with a charity with links to extremist groups.