Mr Cantlie, a photojournalist who has worked for newspapers including The Sunday Times, has featured in several propaganda videos released by the Islamic extremists in recent weeks and apparently had an article published in the group’s Dabiq magazine.
But in a statement, his sister Jessica Cantlie said it was “frustrating” that contact with her family has stopped. “It is not true to say that there has been no attempt to engage with IS. It is simply not accurate,” she said.
“This is frustrating for all parties, including those who are trying to assist us. We had previously been in contact through a channel started by you, but then this stopped for reasons best known to you.
“Sadly, like the families of David Haines and Alan Henning before they were killed, our efforts at re-opening dialogue continue to be ignored by those holding John. We strongly challenge those holding John to return to your previously opened channel, to which we continue to send messages and await your response so that in keeping with everyone’s wishes, we can restart dialogue. “We implore IS to reinitiate direct contact.”
Ms Cantlie’s statement referred to her terminally-ill father, Paul, who pleaded with his son’s captors from his hospital bed.
More needs to be done to stop the militant Islamist group responsible for kidnapping more than 200 schoolgirls in Nigeria exactly six months ago, a letter signed by British politicians and former military leaders has urged.
Boko Haram are now said to be backed by Islamic State (IS), according to the letter published in the Independent newspaper.
Among those calling for an international inquiry and military assistance for Nigeria are former Liberal Democrat leader Lord Ashdown and former Army chief General Sir David Richards.