The message, in template form with a number of details replaced by ‘xxxx', was sent by the agency to several Norwegian media organisations.
“Norway is in mourning. King Harald V has died aged xx. The king died on xxxx (day/date) (home, hospital etc.), at xx:xx (time)”, read the initial alert.
The incomplete text was sent in Norwegian at 12:06pm on Tuesday, with a correction and withdrawal of the obituary sent three minutes later.
“NTB is withdrawing the alert [of the King's death], which was sent due to a technical error. We sincerely apologise,” read the follow-up text.
“It is extremely regrettable that we released an erroneous alert of the King's death. All of us at NTB are very sad that this has happened,” NTB managing editor and administrative director Mads Yngve Storvik said in a message sent out by the agency later on Tuesday, reports Medier24.
“The editorial team has background reports and obituaries for a number of famous people saved in a folder. These are regularly updated. It was in connection with such an update that the alert was sent due to an error,” Storvik continued, adding that a “technical error” appeared to be to blame for the incorrect alert.
“Alerts normally go through several editorial stages before going out on our wires,” Storvik said.
Royal Palace assistant head of communication Svein Gjeruldsen told broadcaster NRK that His Majesty is very much alive and in good form.
“I have heard that NTB released an alert, and I can confirm that the King is in fine form.”
King Harald is thought to be spending part of this week hunting elk, according to NRK.
Erroneous publications of pre-prepared obituaries of heads of state and other high profile figures are not actually that uncommon.
A BBC Urdu journalist faced disciplinary proceedings for a “serious breach” of guidelines in 2015 after sending a tweet mistakenly reporting Queen Elizabeth II had died.
The reporter had seen an internal rehearsal of how the broadcaster might cover a royal death and mistakenly took it to be the real thing.
In 2009, NRK published a video obituary of veteran politician Haakon Lie a week before Lie died.
And last year, Sweden's Princess Birgitta responded angrily to reports published by Vocento, one of the largest multimedia groups in Spain, that she had committed suicide.