UPDATE 1-Tesla strike in Sweden heats up as largest union joins fray

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Country's biggest union joins labour dispute against Tesla

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Unions see the fight as vital to protect labour model

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Tesla and Musk refuse to sign collective agreements

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Unionen may escalate efforts if action is circumvented

(Adds union comments in paragraph 12-13)

By Marie Mannes

STOCKHOLM, May 14 (Reuters) - Sweden's biggest union on Tuesday threw its weight behind a six-month-old strike by mechanics at Tesla, escalating the U.S. carmaker's latest dispute with organised labour.

At the heart of the strike is Tesla CEO Elon Musk's refusal to sign a collective bargaining agreement allowing the metal workers' union to negotiate deals for the workforce as a whole.

Last month, Musk said the labour storm had passed in the country where Tesla's Model Y is the top-selling car, but he was contradicted by IF Metall, which said its strike continued.

The union told Reuters that about 44 of its members - roughly a third of Tesla's Swedish mechanics - had downed tools at the company, which does not produce vehicles in Sweden but services them locally.

"The strike is ongoing and we have no signs of reaching an agreement in the near future," IF Metall head Marie Nilsson said. "We have had a few sittings with the Swedish management during April, but ... Tesla has shown little willingness in discussing an end to the conflict."

More than a dozen unions have launched action in support of IF Metall, with Unionen the latest and biggest.

"It is fundamentally important to protect our collective agreement system," Martin Wastfeldt, head of negotiations at Unionen, told Reuters.

Unionen began a blockade on Tuesday affecting all work for Tesla at DEKRA Industrial AB, which conducts equipment inspections.

If Tesla seeks to circumvent the blockade by hiring other providers, Wastfeldt said Unionen was prepared to do more.

This might involve Unionen members at the company that produces licence plates for Tesla in Sweden, or administrative, human resources and finance staff at Tesla itself.

Access to licence plates has been one focal point of tensions, with Tesla suing unions that have sought to halt postal service to the automaker.

On Tuesday, the union for Service and Communication employees, Seko, said it was taking steps to close a loophole in the blockade being used by Tesla.

Tesla did not immediately respond to a request for comment on the strike. It has previously said its Swedish employees have as good, or better, terms than those the union is demanding.

The conflict is key for the company, whose tough stance globally on unions could be undermined if it buckles in Sweden, or if the strike spreads to bigger markets like Germany.

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While the numbers on strike in Sweden are small, the stakes are high.

Allowing companies to operate in Sweden without collective agreements would undermine unions and threaten the Swedish social model, potentially dragging in the government.

"For IF Metall, it is very important not to lose. They simply cannot do that," trade union expert Anders Kjellberg said.

Sweden's unions take heart from past successes - Unionen signed up payment services group Klarna to a collective agreement last year - and their firepower, with more than 10 billion crowns ($921 million) in IF Metall's strike fund alone.

But while union action has caused some disruption, Tesla's new vehicle registrations in Sweden have broadly kept pace with the market.

Since February, Tesla has brought in about 25 temporary staff from other European countries, some for multiple short-term stays.

While it is unclear whether this is linked to the strike, it contrasts with the previous year, when no such workers were brought in, a review of labour registrations showed. Tesla did not respond to a request to clarify.

Kjellberg noted some possible ways out of the impasse.

Amazon, for example, has a third-party company manage its Swedish warehouses signed up to collective agreements, allowing the U.S parent company to avoid doing so.

"It could last for months or even years, because IF Metall can't give up," Kjellberg said. "But in time, it is possible both parties will want to find a solution."

($1 = 10.8617 Swedish crowns)

(Reporting by Marie Mannes; Editing by Niklas Pollard, Jan Harvey and Mark Potter)