Esther Dingley and Dan Colegate had been traveling all over Europe since 2014.
The 37-year-old Oxford graduate has undertaken a solo hike from Port de la Glere to Port de Venasque, a trek that follows the border between France and Spain, according to local police.
She was last seen on November 22 and was due to return on December 2.
Captain Jean-Marc Bordinaro of the French gendarmerie in Saint-Gaudens told The Times: “Esther Dingley wanted to continue her current lifestyle, her motorhome trips and her sporting activities including hiking, while Daniel Colegate seems a little tired of this nomad. life. ”
He added: “Did Esther Dingley want to go alone to live her life and organize her own demise? Nothing allows us to eliminate this working theory.
Captain Bordinaro told the Daily Telegraph: “It could have been an accident, it could have been voluntary disappearance or a criminal act. We just don’t know, so let’s continue all of these lines for now.
French and Spanish authorities had to cancel searches in the mountains where she was last seen due to heavy snowfall.
Mr Colegate, 38, has been questioned by police on three occasions by officers gathering general information about her. He is not considered a suspect.
A spokesperson for the missing persons charity LBT Global, which assists her partner, said: “There is absolutely no suggestion that (Ms Dingley) is seeking ‘another life’.”
Mr Colegate said in a December 1 Facebook post that the authorities’ “dominant view” was that Ms Dingley was not in the mountains as a result of extensive research.
She is now listed as a national missing person in Spain and her file has been sent to “a specialized judicial unit in France”, he added.
“As it stands, Esther is now listed as a national case of missing persons in Spain and the case has been referred to a specialized judicial unit in France.
“This means they will be looking at other options beyond a mountain accident. ”
Mr Colegate said he was “very grateful” for the considerable efforts of rescue teams in Spain and France, which had used helicopters, dogs and a drone.
“While this is a terrifying development in many ways, I try to focus on leaving the door open for Esther to still come home,” he said in the post.
“She was so happy and cheerful the last time we spoke, I would do anything to see her face and hold her in his arms now.
The couple, from Durham, began traveling after Mr Colegate suffered a serious health problem and documented their campervan adventures online.
A spokesperson for the Office of Foreign Affairs, Commonwealth and Development previously said his staff “support the family of a British woman who went missing in the Pyrenees and was in contact with French and Spanish authorities”.