Patrick Desjardins, the prosecutor in charge of the case, told AFP Tuesday that his team were pursuing a theory that Patricia Wilson, 58, had a brief relationship with the gardener and that her decision to end it led to her death.
"A crime of passion looks the most likely theory but we have not ruled out other avenues of investigation," he said.
The prosecutor said there was evidence that Wilson had told friends that Jean-Louis Cayrou, 50, had acted violently towards her and had, on one occasion, attempted to suffocate her.
Investigators have also established that Cayrou made "an enormous number" of phone calls to Wilson when she returned to England between August 10 and 17 to buy a car.
Wilson has not been seen since being dropped off at her French home in Vabre-Tizac, a picturesque village of 441 inhabitants, by a friend on her return from England on August 17.
Dozens of gendarmes, backed up initially by a helicopter and divers, have been mobilised to search for her without success.
"We are looking for a corpse rather than someone still living," one of them confided.
The heavily wooded local terrain, which is intersected by numerous streams, means the search for a body is effectively like looking for a needle in a haystack. "The chances are that whoever disposed of the body knows the area a lot better than we do," admitted the gendarme.
The local authorities are convinced Wilson was murdered because of large quantities of blood they found around the property and in the gardener's car.
The stains are consistent with Wilson having been involved in a struggle and her body subsequently being dragged into a car.
Cayrou was charged with her murder on Saturday but continues to deny any involvement in her disappearance.
The blood found around the house has been confirmed as having come from Wilson. Police were still waiting Tuesday for the result of tests on samples taken from the car.
Wilson had been living in Vabre-Tizac for several years and began the relationship with the gardener after separating from her English partner last year, according to neighbours.