High winds and six-metre (20-feet) waves made weekend rescue attempts impossible.
Vice-Adm. Emmanuel De Oliveira of the Atlantic Maritime Prefecture said a final effort to attach a towing cable to the 164-meter vessel, which is listing nearly 90 degrees, is set for dawn Monday. If that fails, he said, the vessel will run aground in southwest France by Tuesday night.
Spanish helicopters evacuated the 22-member crew last Tuesday after the Modern Express, carrying 3,600 tons of wood and equipment, sent out a distress call.
He said he expected only "limited" environmental impact if the Panama-registered vessel hits the coast because the cargo was mainly wood and there were so far no signs of leaking of its 300 tonnes of fuel.
Experts from the Dutch company Smit Salavage were called in to try to capture the drifting vessel, helped by two Spanish tugs and a French ship.
It is "totally impossible put the cargo ship upright," De Oliveira said.