The UK Foreign Office has said it is "working urgently to verify" video purporting to show beheading of British hostage David Haines, BBC reported.
The Foreign Office said if true it was another "disgusting murder", and it was offering support to Mr Haines's family.
The aid worker, 44, from Perth, was kidnapped last year in Syria.
Islamic State (IS) militants have already beheaded two US journalists and threatened to kill Mr Haines if US air strikes in Iraq were not halted.
Mr Haines, a father-of-two, was taken hostage in the village of Atmeh, in the Idlib province of Syria, in March 2013.
He had been helping French agency Acted deliver humanitarian aid, having previously helped local people in Libya and South Sudan.
It comes hours after his family had made a direct appeal to IS to contact them on Friday.
The family statement, which was released by the UK Foreign Office and addressed the militants directly, said: "We are the family of David Haines.
"We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us."
IS has seized large swathes of territory in Iraq and Syria and declared a new caliphate - or Islamic state - and killed the two US hostages in recent weeks, posting video evidence on the internet.
The threat to kill Mr Haines was made during a video posted online showing the killing of US journalist Steven Sotloff earlier this month.
The extremist group also killed fellow US journalist, James Foley, last month.