Switzerland, renowned for its clean Alpine air and pristine ski slopes, has named Toyota's Prius the world's greenest car.

The Swiss government gave the Prius the highest green rating in a draft study evaluating the environmental impact of driving around 6,000 different cars, including carbon dioxide (C02) emissions which contribute to global warming.

The Swiss government said it hopes that the list -- which is still in draft form and could be revised before it is finalized next year -- will persuade drivers to buy cars which have the lowest impact on the environment.

The list could also be used to impose an emissions-related tax on cars.

The goal is to modernize the fleet of cars and to make the use of natural resources as efficient as possible, so that using cars will exert less pressure on the environment, said Adrian Aeschlimann, a spokesman for the Federal Office for the Environment.

Toyota's Japanese rival Nissan saw its Patrol GR 3.0 get the lowest rating.

The Prius -- a hybrid car twinning battery power and a combustion engine -- was launched in 1997 by Toyota, the world's biggest car maker. It is the best-selling hybrid car on the road, although hybrids account for only a fraction of global car sales.

Switzerland's NZZ am Sonntag newspaper published the names of the top 20 and bottom 20 cars this week. The Swiss government confirmed the names to Reuters.

Fiat's Punto 1.2 Bipower and its Panda 1.2 Bipower, the Daihatsu Cuore and Citroen's C3 1.4i GNV were all in the top 5 of the draft list, which will be sent to the industry for comments before being finalized.

Ford's Transit and Transit 4x4, the VW T5 and Peugeot's Boxer were in the bottom 5.

Small cars and other hybrid vehicles dominated the top 20 of the list.

The United States accounts for more than half of Toyota's Prius sales. The United States and China are the world's top emitters of carbon dioxide, the main greenhouse gas causing global warming.

Environmentalists welcomed the Swiss government initiative.

It's great when a national government is behind this, and a list like that will have much more sway and can function as a guideline for the automotive industry, said Juergen Resch at Germany's environmental group Deutsche Umwelthilfe.