But Mr Straw stressed the Royal Military Police (RMP) investigation into photographs purportedly showing an Iraqi prisoner being beaten was at a very early stage and that no final judgment on the "awful" scenes depicted had yet been reached.
His comments came as doubts were raised over the authenticity of the photos, published in the Daily Mirror over the weekend.
The paper said they had been received from two soldiers in the Queen's Lancashire Regiment who were horrified at the brutality of a "rogue element" in the British army occupying southern Iraq.
But the BBC quoted anonymous sources close to the regiment as saying that the gun held by a soldier in the pictures was the wrong type of SA-80 rifle and lacked the sling with which it would be worn by serving troops.
A hat seen in the pictures was not of the sort worn by members of the regiment in Iraq, the truck in which the scenes took place appeared to be of the wrong type and the alleged victim showed no signs of the sweat, dirt and injuries that could be expected in an arrest followed by violent interrogation, the sources suggested.
Yesterday morning, the Mirror was standing firmly by its story, which alleged that the Iraqi man had been subjected to an eight-hour beating, during which he was urinated on and had his jaw broken and teeth smashed before being dumped from a moving vehicle. A spokeswoman for the paper said: "We've carried out extensive checks to establish the veracity of the photographs and have no doubts about their authenticity."
Defence sources made clear the RMP investigation would inevitably involve checks on whether the photos were genuine but there were no indications that the inquiry had yet uncovered any reason to doubt them.
Meanwhile, a serving officer claimed that mistreatment of prisoners by British troops was not unheard of in Iraq and that he was "certain" brutality like that seen in the photographs had taken place.
GMTV quoted the anonymous soldier as saying that senior officers did not sanction beatings, but were aware they took place.
"I think there was a nod and a wink to certain things," he said. "They did turn a blind eye to things, well no, not turn a blind eye, there were certain things which people were aware were happening.
"People were literally getting serious, serious beating or kickings when they were plasticuffed you're talking broken ribs, punctured lungs sort of thing."