In the key month of September they were down 35% compared to the same time last year.
Sales of the MG-badged cars dropped by more than 33%. Altogether, MG-Rover sold 11,568 cars in a month when results are traditionally good because it marks one of the now two number plate changes each year.
The MG-Rover share of the market was less than 3%.
The £6,500 CityRover has also failed to take-off, amid a wrangle with the Indian manufacturer TATA, who supply the vehicles which Rover then badge and market.
A Rover spokesman admitted yesterday that the September figures were 'disappointing'. He said they hoped for approval soon from Beijing for a deal with the state-owned Shanghai Automotive Industry Corporation.
Without it, Rover will struggle to build a 'make or break' medium sized family car planned for next year to replace the ageing 45.
The deal would involve the companies sharing the huge costs of developing new models. Professor Garel Rhys, of Cardiff University's Business School, says: "Much depends on the delayed launch of their medium-sized car.
"If that fails, there's nowhere for them to go. They need a big partner to help them out. A new car will give them a breathing space. "But if the new car is not a success, it will only delay the inevitable.'
He believes Rover has less than 12 months to turn itself around.