Ethiopia appears to have lost its final attempt to secure the disputed border village of Badme.
Dusty Badme cost the lives of thousands
The row over the village and its 5,000 residents sparked a two-and-a-half year bloody border war in 1998 that left thousands dead.
Last year, the commission issued a ruling which appeared to award Badme to Eritrea, but Ethiopia appealed saying the Commission had promised that demarcations could be refined.
However, ten days ago, for the first time, the UN Boundary Commission categorically clarified Badme as belonging to Eritrea, throwing out Ethiopia's evidence as "inadequate".
The commission "cannot allow one party to insist on adjustments of parts of the boundary which that party finds disadvantageous", said the 11-page report, a copy of which has just been received by the BBC.
"The maps submitted by Ethiopia were inconsistent as to the location of Badme village and the evidence was nothing like what might have been expected," the report said.
It added that even some of the maps submitted by Ethiopia managed to show Badme village on the Eritrean side of the straight line considered to be the border.
The commission had criticised Ethiopia earlier this month for seeking to change the border ruling after agreeing to accept it as final.
The UN information service, IRIN, said that senior Ethiopian officials on Friday had told them that the ruling would be accepted as "final and binding", but that lawyers were studying the findings.
Eritrea's acting Information Minister Ali Abdu Ahmed welcomed the latest ruling.
He told the UN: "We are talking about the rule of law. No party is allowed to change this. There is no good war and no bad peace".
At the weekend, people in the Eritrean capital welcomed the ruling and told the BBC they hoped this would mean an end to hostilities.
Both Ethiopia and Eritrea agreed to abide by the Hague-based Boundary Commission's demarcation of the border when they signed the Algiers peace agreement in December 2000 ending a bitter war.
Despite awarding Badme to Eritrea, the Boundary Commission has left the door open for changes if both countries deem them necessary.