ECO-VIGILANTES across northern Europe are fighting the growing popularity of 4x4s — by letting air out of their tyres.

They seem to be getting away with it. Having studied the law, the environmentalists concluded that it was legal if the vehicles sustained no damage. Some claim to have let tyres down in front of police officers.

The movement began in Paris late last year and has since spread to other cities in France, Belgium and Holland. Protesters in Italy, Spain and Germany have shown interest in starting similar campaigns.

Now British environmentalists, who adopt a gentler approach, are worried that deflating may become a popular tactic in the UK, alienating mainstream supporters.

The Association des Constructeurs Européens d’Automobiles, Europe’s leading motor trade association, says the number of 4x4s in the European Union more than doubled between 1998 and the end of 2004.

The continental groups compete to see who can let down the most tyres in a night. In December 14 Belgians deflated the tyres of 137 off-roaders.

The most difficult part of the task is to let air out slowly, so the vehicle’s alarm does not go off. To avoid the possibility of owners driving off with flat tyres and putting lives in danger, campaigners leave documents on windows explaining what they have done.

A spokesman for a Paris group, who calls himself Sub-Adjutant Marrant (Joker), argues that drivers of 4x4s do not care that their vehicles emit disproportionate amounts of carbon dioxide, and that politicians are scared of the car lobby. “We emphasise the comic, the burlesque side,” he said. “It would be hard to take us to court. We don’t slash tyres; we deflate them. Air doesn’t cost anything.”

Not everyone is so confident. Protesters in another French city were caught last weekend by the owner of a Mercedes 4x4, who had them arrested. Not content with letting the air out, these campaigners had also smeared mud over the vehicle, to emphasise that it was designed for rural use.

“I spent a few hours at the police station,” said a young member of the group, anxious not to be identified. “I am very afraid of what will happen.”

Protesters in Britain have urged a different approach. “Before the groups in France did this for the first time,” said Sian Berry, of the British Alliance Against Urban 4x4s, “they got in touch with us. They said they’d had a brilliant idea.

“Our initial reaction was that it’s quite amusing, and clever to have established that they aren’t breaking the law. But if just one person needs to go to hospital in a hurry and their 4x4 has a flat tyre, the joke won’t seem so funny. The campaign will be finished.”

The British group seeks to change the minds of the vehicles’ owners by placing spoof parking tickets on windscreens. These contain information about the vehicle’s demerits, written in a gently teasing way.

The tickets were an American idea. Earth On Empty, based in Massachusetts, claims its supporters have issued more than 1m “violation earth” tickets in 500 cities and 48 states.

Another American group put stickers on 4x4s saying: “I’m changing the climate, ask me how.” But when owners found that removing these caused damage to paintwork, the group was sued, and the tactic ended.

Aggressive forms of opposition to large, fuel-guzzling vehicles in America continue to flourish, however. A website, www.fuh2.com, invites people to submit photographs of themselves “saluting” Hummers with an upraised middle finger. Thousands have obliged, including some in Britain.