For some 50 years Germans have considered it a fundamental human right to glue their shoes to the accelerator and hurtle down the country’s autobahns at breakneck speeds.

Now the Social Democrats are taking on a huge political gamble with a pledge to impose a 130km/h (81mph) upper speed limit on all of Germany’s motorways. They are calculating that there has been a sea change in German politics, with more votes to be won by protecting the environment.

The initiative was unveiled at the Social Democrat congress at the weekend, which shifted the party to the left. Led by Kurt Beck, it is the junior partner of Angela Merkel’s Christian Democrats in the country’s governing grand coalition. The congress is being seen as a prelude to divorce; an attempt to get on an election footing.

“We don’t need a return to socialism as dreamt up by the Social Democrats,” said Ms Merkel, the Chancellor. “We had more than enough socialism in the East German state.”

Motorway speed limits are imposed across the continent, but German conservatives see them as an infringement of personal liberty. “The possible benefits are in no proportion to the intrusion on the individual motorist,” said Roland Pofalla, secretary-general of the Christian Democrats.

The clinching conservative argument against speed limits, however, is the damaging effect on the German car industry, which produces big-engined, high-speed vehicles. Drivers from as far away as China travel to Germany to rent a Porsche and take it full pelt down the autobahn.

Ms Merkel - sympathetic to the big-car lobby that helps to power Germany’s exports - will resist any Social Democratic attempt to interfere with speed limits. But there is at least the arithmetical possibility of the Social Democrats making common cause with the Greens and the Left party (both in the Opposition and both in favour of speed limits) to push through the measure. If they did so, it would be the death knell of the Merkel Government, which is supposed to run until 2009.

“If the Social Democrats, the Greens and the Left party are going to impose 130km/h on the country, then BMW, Mercedes, Porsche and the others will abandon their struggle to develop more efficient and environmentally friendly engines,” said the commentator Michael Backhaus in the tabloid Bild yesterday.

The Social Democrats calculate that a speed limit would cut carbon emissions by about 2.5 million tonnes. Germany has about 12,000km (7,500 miles) of motorway, with speed limits on only 5,000km of it. “This is something that will help both the environment and road safety,” said Christoph Matschie, an SPD executive member.

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