The US trade authorities have protested against Japan's nine-month-old scrappage scheme following complaints from Ford, GM and Chrysler that it discriminates against imported cars.

The Financial Times reported that the protest, lodged by the office of Ron Kirk, US trade representative, calls on Japan to review emissions criteria governing which models qualify for the programme.

The report adds that no US-made cars qualify for the Japanese scheme - which has helped the Toyota Prius gasoline-electric hybrid become the top-selling model in Japan this year. While Japanese officials say that the scheme is based on fuel efficiency, Detroit producers are said to blame the means used by Japanese authorities to measure fuel efficiency.

The FT report adds that US irritation was aggravated by the fact that Toyota, Honda and other Japanese producers were among big beneficiaries of America's cash-for-clunkers fleet renewal programme this year.

European brand importers to Japan have also raised objections.

"This is about frankly a bigger issue than the Japanese scrappage program. This is about them starting to deliver the kind of market opportunities that their companies have taken advantage around the world for so long," said Stephen Biegun, Ford vice president for international governmental affairs, in an interview with Reuters.

"Despite being the world's largest exporter of vehicles, with over 2.1 million exported to the US alone last year, Japan remains the most closed auto market in the industrialised world, with less than 5 percent import participation," the American Automotive Policy Council said.

"A renewal of Japan's scrappage program that allows no benefits to US imports strongly confirms the view that Japan maintains an auto market closed to imports," it added.

Reuters reported that Carol Guthrie, a spokeswoman for the US Trade Representative's office, said the Obama administration shared the US automakers' concerns.

In their letter, the US automakers complained that Japan's scrappage program excludes vehicles imported under the so-called "preferential handling protocol," which US companies have used since the 1990s. The protocol is a less costly alternative to Japan's rigorous testing requirements.