The action was conducted by employees of the SAS Ground Handling (SGH) company, which serves airlines including SAS, Danish Air Transport, Eurowings, Icelandair, Qatar Airways and Thai Airways.

Baggage handlers with the company also took similar action – known as a ‘wildcat strike’ when it is outside of agreements between unions and employers – last week, also resulting in delays to flights.

Wednesday’s refusal to work has been deemed by the Danish industrial dispute court Arbejdsretten as being in breach of the collective bargaining agreement [Danish: overenskomst, ed.] between 3F, the union representing the staff, and Copenhagen Airport, Ritzau reported on Wednesday.

“Arbejdsretten has made strong recommendations that work immediately be resumed or strong sanctions may be faced,” 3F Kastrup chairperson Henrik Bay-Clausen subsequently told the news agency.

The cause for the refusal to work is ostensibly anger amongst SGH staff that an employee of the company wanted to join a different union to 3F, which otherwise represents the firm’s workers at Copenhagen Airport.

Newspaper Berlingske reported earlier this week that the individual in question stated in July his intention to join IDA, an engineers’ union, a decision related to his professional background.

SGH colleagues were against that decision, including a local 3F vice-chairperson who was recorded by Berlingske making what appear to be veiled threats over the issue.

“Have you never heard of the law of the jungle. It’s real here. That means there is only one union we join out here. We’re all members of the same union,” the 3F member told his colleague according to the recording obtained by Berlingske.

Bay-Clausen subsequently disavowed the comments made by the SGH employee and 3F member.

“It is absolutely unacceptable to subject colleagues to threats, which is unfortunately what happened. That is not the way 3F works,” he said in a press statement. The person who made the remarks had since stepped down from his role within the union, Bay-Clausen also said.

Nevertheless, SGH workers again refused to work on Wednesday when the union rebel reported for duty.

They later resumed work after their unpopular colleague, who is a temporary staff member, was sent home, according to Berlingske’s report.

The newspaper writes that, according to Bay-Clausen, the baggage handlers refused to work citing “inappropriate” filming of colleagues by the unpopular temp.

SAS has named the union row as the cause for the wildcat strikes, which affected over 6,000 passengers on Thursday and Friday last week.

It may seem bizarre that a disagreement over the union membership of one individual can result in wildcat strikes and delays potentially affecting thousands of airline passengers.

Henrik Jørgensen, an expert on the Danish labour market at Aalborg University, called the SGH scenario an “extreme case” in an interview with newspaper Politiken.

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3F’s position as the union which has the collective bargaining agreement in the sector is key in explaining the dissatisfaction of SGH workers, Jørgensen said.

“This is unusual on the Danish labour market. It’s not normal to have membership of a specific union forced on you,” he told Politiken.

Exclusive arrangements between employers and unions existed until a 2006 law change which made free choice possible – in other words, representation by a specific union cannot be demanded as an employment term.

Prior to that year, unsanctioned industrial protests or 'wildcat strikes' were common against people who bucked the trend and refused to join the union which had made the agreement with an employer or organisation.

Since 2006 “it has not been normal to force someone to be a member of a specific union,” Jørgensen said.

Union membership, although a strong part of Danish working culture, is entirely voluntary.

An argument for joining the union that negotiates and reaches an agreement is that it is this union which deserves support and membership fees from workers who benefit from the conditions it has secured.

“You can argue that. But then you are also calling (other types of union) which are against the (traditional) Danish model illegitimate,” Jørgensen told Politiken.

“They are not working for (that model), but to have it removed. Their members benefit from collective bargaining agreements reached by others because they get the same benefits when a sector is covered by an agreement,” he said.

READ ALSO: Baggage handler wildcat strike could cause delays at Copenhagen Airport