A Danish inventor has been sentenced to life in prison for the premeditated murder and sexual assault of the Swedish journalist Kim Wall in August last year.

The judge, Anette Burkø, and two jurors found Peter Madsen, 47, guilty of all three of the main charges of premeditated murder, aggravated sexual assault and desecrating a corpse.

“The court has been unanimous in coming to this result,” Burkø said. “This is a cynical and pre-planned sexual assault of a particularly brutal nature on a random women who, in connection with her journalistic work, accepted an invitation for a sailing trip on the accused’s submarine.”

The case has gripped Denmark ever since Wall failed to return from a trip on Madsen’s self-built Nautilus submarine on 10 August.

The 47-year-old sat motionless with his eyes closed and his head bowed as the verdict was delivered, a contrast to his behaviour in the trial, when he had looked constantly around the courtroom.

In his final arguments on Monday, prosecutor Jakob Buch-Jepsen argued that Madsen had tortured and then murdered Wall to fulfil his violent sexual fantasies, in a case “so terrible and disgusting that you as a prosecutor have no words to describe it”.

He called on Burkø and the two lay jurors to disregard Madsen’s own account, saying he had changed his story so many times that credibility was “not only low, it is non-existent”.

Wall’s parents were not present in the courtroom for the verdict, although the seven family seats were all taken by other friends and relatives.

Over the course of the 11-day trial, spread over seven weeks, prosecutors presented their case in often horrific detail. They called for a life sentence, which in Denmark averages around 16 years and is very rarely handed down for the murder of a single person.

Defence lawyer Betina Hald Engmark attacked the lack of conclusive physical evidence in the case, with the coroner unable to determine either the cause or timing of death with accuracy. The prosecution, she said, had presented “a horror story without facts”.

Such was the interest in the trial that the Copenhagen district court opened up a special room with a video link on some days to accommodate as many as 115 journalists from 15 countries.

Wall’s parents, Ingrid and Joachim Wall, attended the trial most days and quietly took notes from specially reserved seats beside the media section.

When Madsen was arrested in August, he initially claimed to have dropped Wall off on land, but he almost immediately admitted that she had died on board and that he had then “buried her at sea”.

At first he said her skull had been crushed by a heavy latch, but after the discovery of Wall’s head, he claimed instead that she died in an accident which saw the pressure in the submarine drop, sealing the hatches and flooding the interior with toxic fumes.

The court heard how the injuries inflicted on Wall reflected the content of more than 40 violent video clips found on Madsen’s laptop.

They heard how he had joked about “a murder plan” involving tying up a woman on his submarine and slitting their throat with a knife in a set of messages sent to a friend, which he had subsequently tried to delete.

Four other women testified that Madsen had invited them to join him on the submarine in the weeks leading up to Wall’s death.

Wall was 30 when she died. The journalist had reported from all over the world for Time magazine, the New York Times, and the Guardian, among others. Her boyfriend told the court that she was “incredibly ambitious” and “amazingly curious”.

During the trial, the court heard from two women to whom Madsen had confessed that he might be a psychopath, a judgment backed up by a forensic psychiatric report which described Madsen as a “perverse polymorph” with “psychopathic traits”.

When he was arrested he had flecks of Wall’s blood on his nostrils, scratches on his forearms, and traces of semen in his underpants.

Madsen expressed his regret when asked to make a closing statement at the end of the trial.

“The only thing I want to say is that I’m very, very sorry for what has happened,” he said.

After leaving the courtroom to confer with her client, Madsen’s defence lawyer announced that he intended to appeal the verdict.