The British Foreign Office has, however, continued to express its belief that Mrs Hassan has been murdered based on video footage apparently showing her death at the hands of her abductors in Iraq.
US experts performed dental tests on the body, found by US marines in Fallujah on November 14, and failed to produce an identity match.
US officials said they believed it to be a Western woman.
The unidentified body, clothed in what appeared to be a purple velour dress, was wrapped in a blanket with a blood-soaked black cloth nearby.
A Foreign Office spokesman said that authorities “still believe that Margaret Hassan probably has been murdered.”
“You cannot be absolutely certain until you have a body, which we don’t,” the anonymous official said.
“All the evidence we have is the video footage. As far as we are concerned it demonstrates that she probably has been murdered, but it is not 100 percent conclusive.”
Born in Ireland, raised in England and holding Iraqi citizenship, Margaret Hassan, 59, had lived in Iraq for more than 30 years and worked on projects providing food, medicine and humanitarian aid to Iraqis.
She was taken captive while on her way to work in Baghdad on October 19.
Video images showing a blindfolded woman being shot in the head was received by the Arab satellite broadcaster al-Jazeera on November 16.
The CARE International worker was among the most high-profile foreigners abducted in Iraq this year, and the first female hostage believed to have been killed.
More than 30 male hostages have been executed by their kidnappers.
The Irish government said it has received no new information about the fate of Ms Hassan.