Police are analysing bones found near the spot where missing hiker Esther Dingley went missing late last year.
A mountain runner raised the alarm at around 2pm today after discovering what he believed could be the remains of a body.
Spanish police went to the area but alerted their French counterparts after discovering the spot was just over the border.
French police have now taken charge of the investigation to try to confirm if they are human bones and who they could belong to if so.
Spanish police sources said they appeared to be human bones.
A Civil Guard source said: “A mountain runner has found what could be human bones just over the French side of the border at Puerto de la Glera.
“He called the Civil Guard who went to the area but after confirming it was on the French side of the border they called the French Gendarmerie who went to the spot and have now taken charge of things.
“They appear to be human remains but it will up to French police now to analyse them.”
Another Spanish source said it appeared “90 per cent certain” they were human remains.
Puerto de la Glera - Port de la Glera in France - is close to the 8,796ft Pico Salvaguardia summit where Oxford graduate Esther last made contact with her partner Dan Colegate at around 4pm on November 22 last year.
Specialist officers from Spain and France have carried out several searches of the area around the Puerto de la Glera hiking trail.
Esther’s partner claimed in a recent BBC interview he “could no longer agree” with the idea she had suffered an accident.
He said: “The search has been so prolonged and so intense, that as far as I’m concerned the probability of an accident is now less than the probability of a criminal act.”