Sweden’s national security adviser has announced his resignation as police opened an investigation after he inadvertently left classified documents at a hotel, one of a series of mishaps that Swedish media have revealed in recent weeks.

Henrik Landerholm, whose appointment two years ago sparked debate due to his longstanding friendship with the prime minister, Ulf Kristersson, also left a notebook behind after an interview and left his phone at the Hungarian embassy, incidents particularly embarrassing given his sensitive national security role.

In what is considered the most serious incident, Landerholm left behind classified documents in a safe at a hotel conference centre in March 2023.

“I have been informed that a preliminary investigation has been opened into the incident with the documents forgotten at the hotel conference centre,” Landerholm wrote in a statement. “I have informed the prime minister of this and we have agreed that under the circumstances I can no longer fulfil my work obligations and that is why I’m leaving my position as national security adviser.”

Kristersson had for weeks defended Landerholm’s suitability for the job as the opposition called for his resignation, but on Monday the prime minister acknowledged that the police investigation made it impossible for him to stay on.

“It is not possible for him to continue his work under these conditions,” he said. “The legal process will now run its course.”

According to Sweden’s newspaper of reference, Dagens Nyheter (DN), the documents were found by hotel cleaning staff and a co-worker retrieved them.

DN also reported that Landerholm forgot his mobile phone at the Hungarian embassy overnight in December 2022, at a sensitive time for Sweden’s bid to join Nato, which Hungary was blocking at the time.

In another incident, in January 2023, Landerholm forgot a notebook at the public broadcaster Radio Sweden (SR) after an interview. Instead of a colleague picking up the notebook in person, it was sent in a plastic bag by taxi to a Stockholm cafe, SR said.

Landerholm, 61, a childhood friend of Kristersson, has previously worked as an ambassador, director of Sweden’s Psychological Defence Agency, principal of the Swedish Defence University, and chair of parliament’s defence committee.