But the Daily Telegraph understands Cameron intends to resist calls to recall Parliament this week and instead will wait until after the Scottish referendum vote on Thursday and Friday NZT.
He will then attend the United Nations General Assembly in New York on Wednesday and Thursday of next week, when he will outline plans to combat Isis. MPs have been told to expect to be recalled to Parliament on the Friday after the UN session.
Sources said the Prime Minister was keen not to allow Isis to dictate his timetable for military action.
Henning is a taxi driver who drove a van full of aid to Syria to help children in the war-torn country.
He had taken time off from his job to deliver clothes and bedding that he and friends had collected, when he was abducted on Boxing Day last year.
The 47-year-old father of two was part of a small aid group called Aid 4 Syria. He was on his fourth convoy to the country when he was seized by masked men near the Turkish border.
He is seen in the closing seconds of the horrific two-and-a-half-minute video released on Sunday showing the beheading of Haines.
Henning appears kneeling in the desert, in shapeless orange clothes, as the executioner stands behind him and claims David Cameron "will have the blood of your people on your hands" if he joins an American alliance to fight the militants.
Henning is believed to have been seized when fighters loyal to Isis overran the town of Ad Dana, where he was working with refugees.
He was thrown into a makeshift prison, and initially thought his work for a Muslim charity would save him.
A Syrian activist told a reporter earlier this year that he had spent a night in the same cell, and Henning had seemed in good spirits, believing he was about to be freed. But the activist, who later escaped, found Henning had later been moved to Raqqa, the city considered the capital of Isis.
Friends of Henning described him as a "lovely guy" who had been moved to act by the plight of children caught in the war.
A close friend said: "He was taking over old ambulances, just helping out as much as he could. They were supposed to be over there for about six months but he was kidnapped just a few days after he left."
His wife, Barbara, had been told he was being well treated. The friend said: "He started taking all this Syria stuff seriously when the war started. It was the fourth time he'd gone out there. He had a tattoo saying Aid 4 Syria on his arm. I think he thought it was his mission to help the kids out there."
Debbie Ashton, 47, a neighbour, described him as a "lovely guy".
"He was always asking us if we knew anyone who was throwing their clothes away. He was just trying to help out. He took clothes, bedding, shoes.
"Everyone used to drop it all off at his house. He was really emotional about it all and he used to say those kids need all the help they can get."
