The two teenagers at the centre of the Facebook sex video case dividing Denmark are deeply unhappy at a police decision to revisit the offence, their lawyer has said.

A video shot without permission of the teenagers engaging in sexual acts at a party three years ago, when both were 15, was shared by Danish teenagers on social media in 2015 and early 2016 before gradually falling out of circulation.

But since 15 January, when police announced that they planned to charge 1,004 of those who shared the video on Facebook for distributing child pornography, interest in the video has surged.

“You could say, ‘Aren’t they happy that the police are finally doing something?’,” Helle Hald, a partner at the Sirius law firm in Copenhagen, said. “Well, no, they’re not at all happy. Of course, this affects the victims because people, once again, are talking about this video. I don’t think any of the rest of us can imagine how awful it is.”

Superintendent Flemming Kjærside, who leads Denmark’s cyber-related sexual offences unit, said: “The most important thing is the victims ... They will suffer from this for the rest of their lives.”

The news that teenagers and young adults across 11 of Denmark’s 12 regions and in Greenland face child pornography charges, despite most of them being 15 or 16 at the time they shared the video, received extensive coverage in Denmark and abroad.

Those found guilty are unlikely to receive a prison sentence but, if convicted of distributing child pornography, it will remain on their records for at least 10 years, preventing them from obtaining the “children’s certificate” required to work as a teacher, scout leader, sports coach, police officer or in other roles involving minors.

“It’s the world’s dumbest thing,” Mira Bech, 19, one of those being charged, told Denmark’s TV2 channel. “There are so many who have had this video who have not been called in for questioning. My friends wonder why it’s only me who’s been questioned. It’s ridiculous.”

Preben Bang Henriksen, who speaks on legal matters for the governing Liberal party, told the Politiken newspaper that he would consider moves to change the law if the teenagers were prosecuted for child pornography. His proposal has won the support of at least three other parties, with only the opposition Social Democrats supporting the law as it stands.

“A lot of parents are complaining, and I guess I would have done the same if someone from my family was involved,” Kjærside said. “But I can’t stop an investigation because I feel sorry for those involved. By law, we are obliged to investigate.”

After the girl in the video went to the police in 2015, the two teenage boys who filmed and shared it were prosecuted and fined 2,000 Danish kroner (£235), making them immune from charges under the police’s Operation Umbrella.

Helle Hald said that in 2016 she had tried to push local police in the area north of Copenhagen, where the offence took place, to investigate and charge other teens who had shared the video, but had not succeeded, despite providing evidence of the video being shared.

Police revived the case late last year, when Kjærside’s unit was contacted by Europol and handed data collected by Facebook detailing all the Danish citizens who had shared the video.

Facebook detected US citizens sharing the video and passed the data to the US National Center for Missing and Exploited Children, who passed it to the Department of Homeland Security, who contacted Europol.

Kjærside said this was the first time in Europe that police had made a concerted effort to tackle the widespread and growing problem of teenagers sharing sexual videos and pictures of each other, often without permission.

“This is not only a Danish phenomenon. It’s also known in the UK, Germany, Italy,” he said. “Everybody has this problem, but none of them, so far as we know, has made such a big effort as we’re doing.”

He welcomed the media attention that the operation has brought, saying that it was finally getting through to teenagers that sharing sex videos without the consent of those filmed is a crime. “We are running campaigns all the time about this, but nobody listens,” he said. “This time we’ve really got their attention.”

The first 10 cases, selected to represent different levels of severity of offence and likelihood of conviction, are expected to go to trial next week.