Muslim groups across Britain united on Saturday to join Prime Minister David Cameronin condemning the beheading of aid worker Alan Henning by Islamic State insurgents, with one leading cleric calling it a "despicable and offensive act".

Prayers for the 47-year-old taxi driver from Salford in northern England were said in mosques throughout the country at the start of the Muslim Eid al-Adha festival.

Cameron called Henning a gentle, compassionate man who had simply tried to help others. Britain would do all it could to destroy his killers, he said.

Speaking after meeting the heads of Britain's armed forces and intelligence agencies, Cameron said in a broadcast message: "we will use all the assets we have ...to defeat this organization which is utterly ruthless, senseless and barbaric in the way it treats people."

Henning had been held captive in Syria for nine months before a video was posted on YouTube on Friday showing him kneeling before a masked knife man against a desert setting.

The masked man spoke briefly with the same southern British accent as that of the killer of previous hostages widely dubbed "Jihad John".

A second video featuring an unmasked, apparently British fighter pouring scorn on Cameron for failing to send in ground troops was being urgently examined by police on Saturday.

Henning was the fourth hostage to have been beheaded by Islamic State (IS), which has faced air strikes by U.S., British, French and Arab fighter jets since seizing swathes of Iraq and Syria.

His case had prompted a wave of appeals for his release from British Muslim leaders and on Saturday morning several expressed their shock at the murder.

Shuja Shafi, Secretary General of the Muslim Council of Britain, tweeted: "Saddened by reported murder ofAlan Henning. A despicable and offensive act. He helped Muslims. My thoughts and prayers with his family."

A group calling itself Muslims of the North of England called Henning a "national hero" while Mohammed Shafiq, chief executive of the Ramadhan Foundation that aims to help young Muslims, said: "This barbaric killing is an attack against all decent people around the world."

AIR STRIKES

Henning had been part of an aid convoy taking medical supplies to a hospital in northwest Syria in December last year when it was stopped by gunmen and he was abducted.

Fears for his safety had grown since the British parliament voted last month to take part in air strikes against Islamic State in Iraq.

In the YouTube video he appears to read from a script before being killed. "Because of our parliament's decision to attack the Islamic State, I, as a member of the British public, will now pay the price for that decision," he says.

Britain's Muslim leaders have in the past been criticized for what some critics have said is a lack of willingness publicly to confront what Cameron has called the "poisonous ideology" of Islamic extremists.

But the case of Henning, who had taken unpaid leave and left behind his wife and two teenage children to help Muslims deliver aid to children in Syria, had prompted a united response.

Last month, a letter signed by over 100 British Imams and Muslim leaders condemned Islamic State.

"The despicable threats to Mr Henning at the hands of so-called 'Muslims' cannot be justified anywhere in the Quran and the Sunnah (Prophetic traditions)," it read.

"The un-Islamic fanatics are not acting as Muslims, but as the Prime Minister has said, they are acting as monsters," it added. "They are perpetrating the worst crimes against humanity. This is not Jihad (Holy war) - it is a war against all humanity."

Henning's wife Barbara had appealed for his release.

"Alan is a peaceful, selfless man," she said. "When he was taken, he was driving an ambulance full of food and water to be handed out to anyone in need. His purpose for being there was no more and no less."

On Friday Paul Cantlie the father of another Briton being held captive by Islamic State, journalist John Cantlie, issued his own appeal for his son's release.

Cameron said Islamic State had paid no heed to any appeals and paid tribute to Henning's desire to help others.

"He went with many Muslim friends out to do no more than simply help other people. His Muslim friends will be mourning him at this special time of Eid," he said.

His office later issued a statement saying Cameron had raised the issue of the second video.

"The police are urgently investigating the contents of the video, including possible terrorism offences relating to it," it added.