First they kidnapped their bosses; then they threatened to blow up their own factories. Now, in the latest phase of France's summer of discontent, disgruntled workers are turning to environmental blackmail as a stick to beat the management into submission.

Angry lorry drivers at Serta, a struggling transportation company, are threatening to pour more than 8,000 litres of toxic fuel additive into the Seine if their demands for redundancy pay-offs are not met. Acknowledging the "dramatic" effect this could have on the river's fish population, they insist they will not be dissuaded unless their bosses give in.

"It's less dramatic than ... people being made redundant and sacrificed," Jean-Pierre Villemin from the CFDT union told French radio. "It's the only means we have of getting what we want."

Around 50 workers at the distribution site at La Vaupalière near Rouen are demanding severance packages of 15,000 euros after Serta, which went into administration a year ago, announced job cuts. The transportation company, which has suffered badly in the financial crisis, has already cut around 80 jobs since the start of the year.

Their threat to flood with the harmful substance their on-site drainage system - designed to channel rainwater back into the Seine - is the latest tactic used by workers desperate to draw attention to their plight.

Last month, workers at New Fabris, a bankrupt car parts plant, and at Nortel, an insolvent telecommunications company, vowed to explode gas cylinders at their factories if requests for improved severance package were not met. Both threats have since been lifted.

These actions, decried as media stunts by their critics, followed a springtime spate of so-called "boss-nappings" across France in which business executives were taken hostage by their bellicose employees. Such episodes are familiar features in the country's sociopolitical landscape and received more attention abroad than they did at home.

The more recent threats of environmental damage, however, are more unusual.

It may be that the Serta drivers are seeking to recreate the success of an infamous workers' campaign nine years ago in which workers at the Cellatex textile plant poured thousands of litres of sulphuric acid into the river Meuse. They were rewarded by management with a year-long redundancy package of 80% of their salary.

"The workers ... do not want to leave with the frankly pathetic minimum legal compensation," said Villemin, who has been on strike with the workers since last week. "If we do not obtain decent pay-offs we will unfortunately be reduced to opening the [fuel] drums and pour the contents into the sewers."

Antoine Faucher, campaign director of Greenpeace France, said the threats, though worrying, were in fact a reflection of growing concern for the environment. "It's significant because today, perhaps unlike previous years, the environment is recognised in itself as a resource," he said. "To take it hostage may be of greater value now than it was before."