Police are searching for a gang of radical environmental activists after a series of attacks on 4x4 vehicles.

The gang, who claim to have targeted up to 80 vehicles across South Manchester, let down tyres and leave notes accusing the owners of adding to global warming and increasing the chances of road deaths.

In the last week tyres on 20 vehicles were slashed or deflated in the Ladybarn and Withington of the city. This follows similar attacks on 11 cars last month. Police classify the deliberate acts as criminal damage.

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A statement from the activists said tyres were deflated rather than slashed. It added: "Given the threat of climate change and the Government's inaction, direct action such as this is, unfortunately, necessary. Large SUVs (Sports Utility Vehicles) emit substantially more greenhouse gases."

The statement also said accident victims were six times more likely to die if hit by an SUV rather than a smaller car.

Owners of SUVs – dubbed Chelsea tractors – have been branded irresponsible by environmentalists, for their vehicles' size and fuel consumption. Critics say the large four-wheel drive vehicles were originally intended for use by farmers on rough terrain in the countryside. But they have become popular with middle-class families living in cities and are used for school runs and shopping trips.

Patricia and Waris Ashraf had a tyre let down on their Mercedes ML outside their house in Whalley Range, making their daughter Natasha, 16, late for an A-level exam. A victim from Chorlton, who would not be named, said: "What concerns me most is that we have a child seat in the back of the car. They must have seen that, which suggests they don't care who they affect."

Doug McMillan, whose tyres were attacked in Parrs Wood, Didsbury, last week, said: "If they ever did have a noble, credible cause, they have blown it because they have reduced themselves to the status of common criminals. They are vandals, nothing else."

A member of the group calling himself James said: "These vehicles are totally unsuitable for the city, they're dangerous, polluting and an unnecessary status symbol. They should not be on our city roads."

Detective Inspector Damian Moran, from Greater Manchester Police, said: "Those responsible might believe they are making a point, but this behaviour is criminal.

"It is mindless vandalism with no regard for the distress and nuisance caused to decent members of our community and will not be tolerated.

"If anyone knows anything or has seen anything suspicious during those two nights that might help us catch those responsible, please contact me."