Prime Minister David Cameron has said Britain “stands united with Japan” after the Islamic State (IS) group released a video appearing to show the beheading of hostage Kenji Goto.
Freelance journalist Mr Goto, 47, is seen kneeling in front of the British-accented figure known as Jihadi John before footage of his decapitated body is shown.
It comes less than a week after news of the beheading of another Japanese man, Mr Goto’s friend Haruna Yukawa. IS, also known as Isil, had demanded a £130 million ransom for the two men.
Mr Cameron said: “I condemn what appears to be the despicable and appalling murder of Kenji Goto. It is a reminder that Isil is the embodiment of evil, with no regard for human life.”
In the video, a masked man looks into the camera and says: “To the Japanese government: You, like your foolish allies in the Satanic coalition, have yet to understand that we, by Allah’s grace, are an Islamic caliphate with authority and power, an entire army thirsty for your blood.
“[Japanese prime minister Shinzo] Abe, because of your reckless decision to take part in an unwinnable war, this knife will not only slaughter Kenji, but will also carry on and cause carnage wherever your people are found. So let the nightmare for Japan begin.”
Japan’s deputy foreign minister Yasuhide Nakayama said earlier that efforts to free Mr Goto had been “in a state of deadlock”.
The Japanese government initially said it was trying to authenticate the video.
Mr Cameron added: “My thoughts and prayers are with the families of Mr Goto and Mr Yukawa and the Japanese people as they come to terms with the murder of two innocent citizens in such a brutal manner.
“Britain stands united with Japan at this tragic time and we will do all we can to hunt down these murderers and bring them to justice, however long it takes.
“I welcome prime minister Abe’s steadfast commitment to continue Japan’s active role, working with international partners, to secure peace, stability and prosperity in the Middle East.
“The humanitarian aid they are providing in the region is a vital part of helping the local communities that are being persecuted by the same Isil terrorists who murdered our innocent men. The Japanese government is right not to bow to terrorism. The way we will defeat Isil is not by giving in to these terrorists but by confronting them and their poisonous ideology.”
“With determination and patience, we will work together with Japan and our other allies to extinguish this terrorist threat and to stand up for the values of tolerance and peace.”
Mr Goto, who is also a film-maker, went to Syria in October, reportedly to try to secure Mr Yukawa’s release.
No mention was made in the footage, titled A Message To The Government of Japan, of the fate of Jordanian pilot Muath al- Kaseasbeh.
The militants had threatened to kill him unless failed suicide bomber Sajida al-Rishawi was handed over by Jordan by sunset on Thursday.
Bernadette Meehan, a spokeswoman for the White House’s National Security Council, said: “The United States strongly condemns Isil’s actions and we call for the immediate release of all the remaining hostages. We stand in solidarity with our ally Japan.”
Mr Goto’s mother Junko Ishido last week issued a tearful appeal for her son to be released.
She said: “I say to you people of the Islamic State, Kenji is not your enemy. Please release him.”
Mr Goto’s death is the latest in a growing list of foreign hostages apparently killed by Jihadi John.
It includes British aid workers David Haines, from Perth, and Alan Henning, from Salford, American journalists James Foley and Steven Sotloff, and American aid worker Peter Kassig.