COPENHAGEN (Reuters) -Sweden's largest insurer Folksam said on Wednesday it has sold its stake in electric car maker Tesla over concerns about the company's stance on workers' rights.

Folksam said in a statement it had tried to influence Tesla, together with other owners, through proposals submitted to the company's annual general meeting two years in a row.

"Unfortunately, no improvement has been seen and a decision has therefore been made to divest the holding," its head of asset management and sustainability Marcus Blomberg said.

It was not immediately clear how many shares Folksam had sold.

The U.S. electric vehicle maker headed by billionaire Elon Musk is at the centre of a dispute in Sweden over its refusal to sign a collective bargaining agreement and thus allow a labour union to negotiate on behalf of workers.

Tesla did not immediately respond to a request for comment.