The murder of East Yorkshire-born aid worker David Haines was an "act of pure evil", Prime Minister David Cameron has said.
The Foreign Office said it was "working urgently" to verify a video released by Islamic State militants on Saturday night, which appeared to show Mr Haines being beheaded.
Mr Cameron said the government would do "everything in our power" to hunt down Mr Haines' killers.
In a statement released by Downing Street, he said: "This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil.
"My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal.
"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."
Father-of-two Mr Haines, 44, was born in Holderness before moving to Perth. He has been widely praised for his humanitarian work.
He was kidnapped from a Syrian refugee camp last year, where he was helping to deliver humanitarian aid.
In the video, a militant with an English accent blames the death on Mr Cameron for "arming the Peshmerga (Kurdish armed forces) against the Islamic State". It includes a threat to kill another hostage, said to be British.
US President Barack Obama condemned the murder. He said: "Our hearts go out to the family of Mr Haines and to the people of the United Kingdom.
"The United States stands shoulder to shoulder with our close friend and ally in grief and resolve."
The release of the video came just hours after Mr Haines' family had made a direct appeal to IS militants - who have murdered two American hostages - to contact them.
In a statement released by the Foreign Office, Mr Haines' brother Mike paid tribute to "just another bloke" who was "was most alive and enthusiastic in his humanitarian roles".
He said: "David was like so very many of us, just another bloke. Born in 1970 to parents who loved us both, our childhood was centred around our family.
"Holidays in caravans and tents, days away as a family which we remember fondly. David and I were brought up to know right from wrong, although we might not with the innocence of youth have always chosen right.
"David was a good brother, there when I needed him and absent when I didn't. I hope that he felt the same way about me.
"He was, in the right mood, the life and soul of the party and on other times the most stubborn irritating pain in the ass. He would probably say the same about me.
"After leaving school he worked with the Royal Mail before joining the RAF as an aircraft engineer. He married his childhood sweetheart Louise and in the due process of time had a wee lass Bethany.
"He was - and no doubt wherever he is - exceptionally proud of Bethany.
"David served with the UN in the Balkans, helping people in real need. There are many accolades from people in that region that David helped. He helped whoever needed help, regardless of race, creed or religion.
"During this time David began to decide that humanitarian work was the field he wanted to work in.
"David left the RAF and was employed by Scotrail. As with every job, David entered into it with enthusiasm. David met and married his second wife Dragana and they have a four year old daughter Athea.
"David was most alive and enthusiastic in his humanitarian roles. His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair.
"He was and is loved by all his family and will be missed terribly."
