The battalion at the centre of controversy over video footage showing soldiers beating Iraqi youths was named tonight as the 1st Battalion The Light Infantry, which won an award last year for its service in southern Iraq.

Royal Military Police this evening confirmed that they are questioning members of the unit as they investigate the fresh abuse allegations, which have raised fears of a backlash against British troops.

Investigators last night arrested a man in connection with the inquiry.

A spokesman for the Ministry of Defence said today that the allegations, which surfaced in a short video tape published in the News of the World yesterday, were "extremely serious".

The 1st Battalion The Light Infantry (1LI) has taken part in three deployments in southern Iraq. The regiment was one of 14 to be awarded a theatre honour and battle honour in November, 2005. A Ministry of Defence spokesman said that the battalion was assisting with inquiries as it attempted to identify all personnel captured in the film.

The investigators have contacted commanding officers of all units deployed in Iraq in 2004. The video footage matches descriptions of riots in Amarah in January 2004, when British troops drove back a crowd who began protesting after queuing for jobs in the new Iraqi Civil Defence Corps, only to be told that there were no vacancies

Three people died. A British Army spokesman said at the time: "One, maybe two [of the dead] were possibly killed by UK troops. Those troops were firing in self-defence. It was quite clear that a number of objects were thrown at the UK troops, possibly grenades."

The naming of 1 LI suggests that it was filmed during the deployment of 20 Armoured Brigade, based in Paderborn, Germany, which was in southern Iraq until April 2004. As well as 1 LI, the brigade included the 1st Battalion the Royal Regiment of Wales, the Queen’s Royal Hussars and 26 Royal Artillery.

Ibrahim al-Jaafari, the new Iraqi Prime Minister, has called for a rapid investigation.

Tony Blair has promised that the allegations would be investigated "very fully indeed" but said that the overwhelming majority of British forces behaved properly and deserved public support.

The evidence is contained in a three minute, 12 second video tape that was handed to the News of the World. It shows the beating of the four young Iraqi males, including a child, which followed a riot in southern Iraq in early 2004. In addition, another defenceless man is filmed being kicked by soldiers and a civilian corpse is defiled. The beatings are accompanied by a gleeful commentary from a corporal who allegedly filmed the attacks.

Arab satellite television stations, including Al-Jazeera and Al-Arabiya, today replayed the footage alongside images from the Abu Ghraib prisoner abuse scandal involving American soldiers.

There had been no complaint to the authorities from any of the assaulted Iraqis but the Special Investigations Branch (SIB) of the Royal Military Police is trying to trace them.

The MoD has urged all those involved to give themselves up to prevent other regiments being tarnished by speculation.

"The images in this video amount to very serious allegations," said Brigadier Martin Routledge, the Adjutant General's Chief of Staff. "We can confirm they are now the subject of an urgent Royal Military Police investigation. We condemn all acts of abuse and brutality, and always treat any allegations of wrongdoing by our personnel extremely seriously."

Organisers of a huge march in London planned by Muslims for this Saturday said that the newly-disclosed abuse would fuel the anger caused by cartoons of the Prophet Muhammad.

There were predictions that British troops would feel the backlash in Iraq. Sir Iqbal Sacranie, Secretary-General of the Muslim Council of Britain, said that the images posed a danger to 8,000 soldiers already in Iraq and the 3,000 pledged for Afghanistan.

"Incidents like this cause enormous damage to our standing in the Muslim world and also place those British troops who our carrying out their duties conscientiously in greater peril," Sir Iqbal said. "It is vital that all those soldiers involved in this abuse of the Iraqis are identified speedily."

All British units in Iraq were put on alert. Major Peter Cripps, an Army spokesman in Basra, said today that there had not yet been any recriminations on the streets of the southern Iraqi city, where most of Britain’s more than 8,000 troops are based.

But Basra City Council chief Mohammed al-Abadi said local leaders wanted quick action and assurances that Iraqis would not be "humiliated further". "We condemn and denounce this criminal and brutal act. Iraqis don’t deserve such treatment," he said.

All political parties were united in their condemnation of the alleged footage. Michael Moore, the Liberal Democrat defence spokesman, wanted to know if the alleged abuse was "a one-off, as hoped, or evidence of wider abuses". Liam Fox, the Shadow Foreign Secretary, said that "British troops must adhere to the highest possible standards" but asked that the actions of a "tiny minority" did not taint the armed forces’ reputation.

The MoD was unaware of the incident until the video was handed over on Friday. SIB was immediately called in but after examining the footage was unable to identify the unit. Defence sources said that the newspaper had given the investigators the name of a unit, but that it was not clear whether this was the right one.

In 2007, 1 LI is to be amalgamated with the Devonshire and Dorsets, The Royal Gloucestershire, Berkshire and Wiltshire Regiment (RGBW), the 1st and 2nd Battalions of The Royal Green Jackets and the 2nd Battalion Light Infantry into five battalions of a formation to be called The Rifles.

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