Elon Musk has decried a wave of “insane” strikes focused on Tesla factories in Sweden, as workers target the US electric car manufacturer in a strike calling for collective bargaining rights.

In what has been portrayed as the largest fight to save Sweden’s union model from global labour practices in decades, the powerful trade union IF Metall has been leading a strike across eight Tesla workplaces in Sweden for five weeks.

It is the first time workers for the US carmaker have gone on strike and on Thursday, Musk, the tech billionaire and chief executive of Tesla, made his feelings clear, writing on X, formerly Twitter: “This is insane.”

He was responding to a social media post about secondary, or sympathy, strikes by Swedish postal services that are preventing licence plates reaching new Tesla cars.

IF Metall, which has more than 300,000 members in Swedish industry, has said it will “keep going [with the strikes] for as long as needed”. It says it took action after Tesla refused to sign a collective agreement with its members.

Collective agreements, which cover conditions including salary, pension, working hours and holidays and mean that, in theory, unions rather than the state govern the labour market, are seen as a cornerstone of Sweden’s labour market model.

Although union membership in Sweden has fallen in recent decades, many workers are still in unions and about nine in 10 employees have collective arrangements.

The Tesla strike has attracted secondary action from eight other unions and is threatening to spread to neighbouring Norway, where Fellesförbundet (the United Federation of Trade Unions), the country’s largest private sector union, said it was prepared to take sympathy action.

Marie Nilsson, chair of IF Metall, said the strike was not only a fight for Tesla workers, but to protect the Swedish union model. “If we allow companies like Tesla to operate without collective agreement, it will open it up for other international companies and other types of industries,” she said. “It can take a long time,” she added. “We will keep going for as long as is needed.”

The strike has gained support from transport and harbour workers, who have refused to load or unload Tesla cars in all Swedish ports; electricians who have refused to carry out service or repair at Tesla’s workshops; and charging stations and painters, who will not work on Tesla cars. Other sympathy strikes include those by service and communication workers, who have stopped distributing post and shipments to Tesla.

“We are well prepared for a prolonged conflict,” said an IF Metall spokesperson, Jesper Pettersson. “Regrettably, we have no ongoing talks with Tesla Sweden at the moment, but as always, we are available for further talks as soon as possible.”

The action appears to be influencing businesses beyond Tesla. Shortly after the Tesla strike began, the Swedish global payment firm Klarna signed a collective agreement, averting a planned strike at its Stockholm headquarters.

Afterwards, Klarna’s co-founder and CEO, Sebastian Siemiatkowski, made reference to the Swedish model, saying: “I am convinced that we will benefit from this agreement and that Klarna can contribute to making the Swedish model stronger from the inside.”

Some commentators have suggested that the action at Tesla could start conversations at the Swedish division of Spotify, the streaming firm, which earlier this year pulled out of talks about a collective agreement, saying it did not believe that would “add any significant value” for employees.

In Norway, where Fellesförbundet says about 500 Tesla employees are organised, the union leader Jørn Eggum said it would block Swedish Teslas from coming to the country. “Norway should not be a transit country for Tesla to get away with strike-breaking,” he told broadcaster NRK earlier this month. We must hold Tesla accountable and make them commit to collective agreements in the European countries in which they operate.”

Jesper Hamark, an economic history researcher and visiting research fellow at the University of Gothenburg, said the strike was about defending the Scandinavian model against the American one.

Comparing it to strikes in 1995 at Toys R Us, which the unions won, he said: “My guess is that Tesla will not remain in Sweden without collective arrangement. The union will win. I have difficulty seeing the unions folding. The question is too important.”

Tesla and Spotify did not immediately respond to requests for comment.