Just 922 Rover cars were sold in the UK in August, 37% less than in the same period last year, motor industry figures show.
MG Rover hope sales pick up in September
Parent company MG Rover had just under 2% of the UK market share in August, compared with Vauxhall's 15%.
The firm's MG marque sold 764 cars last month, 35% less than in August 2003.
Rover said the data was disappointing but pointed out that sales are usually slower ahead of this month's registration number plate change.
"August is a traditionally a dip month," said Nigel Wonnacott, spokesman for the Society of Motor Manufacturers and Traders (SMMT).
"Because of the number plate change, September registrations are likely to be four or five times those of August," he said.
Market share down
Overall, UK car registrations were down 4.86% last month compared with August 2003, the SMMT figures show.
In the eight months to the end of August, registrations had risen by nearly 1%, compared with the same period last year.
A total of 30,986 Rovers have been registered this year, down 19.56% on 2003. MG registrations totalled 21,045, down 7.97% on last year.
MG Rover had a 3.16% share of the UK market in the first eight months of 2004, compared with 3.76% in the same 2003 period.
UK car sales share Jan-Aug 2004
MG Rover 3.16%
The year-to-date figures show Ford's share of the UK car market is 14.68%, with sales down 2.3% on the same period last year, and Vauxhall's share is 12.93%, with sales up 2% on last year.
Other companies to see sales fall in the year to August, compared with the same period in 2003, include Citroen, Nissan, and Mercedes.
MG Rover has recently denied speculation that it is to be taken over by China's largest automaker Shanghai Automotive Industry Corporation (SAIC).
A report by Automotive News Europe quoted UK and Chinese sources as saying SAIC would take over Rover.
In June this year, MG Rover signed a technology-sharing agreement with the Chinese firm.