A public inquiry into claims that British troops murdered and tortured Iraqi detainees nine years ago was shown graphic photographs of alleged victims as it opened in London on Monday.

The Al-Sweady Inquiry, named after one of the dead men, 19-year-old Hamid Al-Sweady, will examine allegations British soldiers unlawfully killed detainees after a gun battle at a checkpoint in Maysan Province, southern Iraq, in May 2004.

It will also look into allegations that detainees captured at the same time were mistreated at a British base, Camp Abu Naji (CAN), and at a detention facility at Shaibah Logistics Base between May and September that year.

Britain's Ministry of Defence (MoD) vigorously denies the allegations.

The inquiry, first ordered by the British government in 2009, is being chaired by a former High Court judge, Thayne Forbes, and began with an opening statement from counsel to the inquiry Jonathan Acton Davis.

Acton Davis said the inquiry, which is being held at Finlaison House, a government building in central London, aimed to identify the circumstances of the deaths of 28 Iraqi men.

The inquiry was shown photographs of the dead, and Acton Davis said their death certificates gave causes of death including gunshot wounds, signs of torture, broken bones and mutilation, while several had missing eyes and one man's penis was missing.

Acton Davis said the inquiry was ordered after five Iraqi men claimed at London's High Court that their human rights had been violated.

"The claim related to events which began on May 14, 2004, when Iraqi insurgents ambushed vehicles belonging to the Argyll and Southern Highlanders near to a permanent vehicle checkpoint known as Danny Boy which was some five kilometres north-east of Majar Al-Kabir on Route Six in Iraq," he said.

"A fierce battle ensued which involved not only the Argylls but also soldiers from the Princess of Wales Royal Regiment.

"It resulted in many Iraqis being killed and in two British soldiers being wounded."

He said enemy dead would usually have been left on the battlefield, but British soldiers were apparently given an order to identify the dead to try to find a man thought to be involved in the murder of six British soldiers in 2003.

As part of that order, the dead Iraqis' bodies were taken back to Camp Abu Naji, and nine men were detained, he said.

"It was the claimants' case that not all of the 20 died on the battlefield, and that at least one of them was murdered by a British soldier after he had been returned alive to CAN," Acton Davis said.

It is claimed that Hamid Al-Sweady was killed at Camp Abu Naji either on May 14, 2004 after he had been detained, or on May 15 before his body and that of 19 other Iraqis were handed back to local Iraqi authorities.

Acton Davis said there was a "stark dispute" between Iraqi and British military evidence.

An MoD spokesman said: "The MoD takes all allegations of abuse extremely seriously, which is precisely why we decided that the Al-Sweady allegations should be independently investigated.

"This public inquiry has our full support and we will co-operate with it fully."

Some 15 Iraqis will travel to Britain to give evidence to the inquiry later this month, including Al-Sweady's uncle Khuder Al-Sweady and several other detainees.

The inquiry team has already taken statements from Iraqi witnesses in Beirut and Istanbul, as well as from military witnesses, and has trawled through large amounts of evidence.

It is the second public inquiry into allegations of abuse by British soldiers in Iraq, following one that concluded in 2011 which examined the death of an Iraqi man, Baha Mousa, in 2003.

That inquiry found that Mousa was hooded, assaulted and held in stress positions along with nine other Iraqis following their detention by 1st Battalion the Queen's Lancashire Regiment in September 2003.