Paul Watson, the anti-whaling campaigner, has spoken of delight that he will be reunited with his young children for Christmas after Denmark rejected Japan’s extradition request and released him from prison in Greenland.

After 150 days in jail on the Danish autonomous territory, Watson, 74, was told by his lawyer on Tuesday morning that the Danish authorities had decided he was free to leave the island.

Watson told the Guardian: “At 8.46 this morning a guard came – I had just woken up – and said the lawyer’s on the phone.

“I am certainly relieved as this means I get to see my two little boys. That’s really been my only concern this entire time. I understand the risks of what we do and sometimes you get arrested – although I am proud of the fact that I have never been convicted of a crime.”

Watson, who has two sons aged three and eight, was apprehended in Nuuk, the capital of Greenland, on 21 July by a dozen police officers while refuelling his ship, MV John Paul DeJoria. The police were acting on an Interpol red notice issued by Japan.

Tokyo was seeking his extradition on charges relating to the alleged boarding of the Japanese whaling ship Shōnan Maru 2 in the Southern Ocean in February 2010. The charges, including one of assault, carried a sentence of up to 15 years in jail.

Watson, a pioneer behind the founding of Greenpeace who has spent five decades thwarting whalers on the high seas, was not at the scene of the alleged crimes and had denied the allegations.

Japan voiced dismay at his release. “It is regrettable that the Denmark government did not accept Japan’s request of passing him over and [the government] has conveyed this to the Danish side,” said top government spokesman Yoshimasa Hayashi.

Explaining the decision, Peter Hummelgaard, Denmark’s minister for justice, said he had not received sufficient assurances from Japan that Watson’s five months in jail awaiting extradition would be deducted from any future sentence.

He said: “The decision is based on an overall assessment of the case. In light of the total amount of time that Paul Watson would be expected to be detained until a possible decision on extradition could be implemented, and taking into account the nature and age of the offences, it has been of particular importance for the Danish Ministry of Justice to ensure that the time Paul Watson has been detained in Greenland will be fully deducted from a potential custodial sentence that he might be sentenced to in connection with the criminal case in Japan.

“Based on the correspondence with the authorities of Japan regarding this matter, the Danish Ministry of Justice has assessed that it cannot be assumed with the necessary degree of certainty, that this would be the case.”

Hummelgaard said that criticism of Japan’s legal system had not been part of his consideration.

He said: “Japan is a democratic country guided by the rule of law. There has been a good and close dialogue with the authorities of Japan during the processing of the case.”

A campaign for his release had won the support of a range of high-profile people, including the French president, Emmanuel Macron; Brigitte Bardot; and the Irish actor Pierce Brosnan.

Watson said: “My time in jail has at least served the purpose of focusing attention on Japan’s illegal whaling operations and the added benefit of focusing attention of the killing of dolphins on the Danish Faroe islands. So in that sense it was a successful campaign.”

He added of his time in jail: “My position is that you prepare for the worst and hope for the best, that’s all you can do. One day I would feel confident and then others, well, you know, Japan was putting a lot of economic pressure on Denmark. Fortunately, Denmark has got a good record on human rights.”

In a prison cell interview with the Guardian earlier this month, Watson had said he did not believe he would survive a spell in a Japanese jail. “I know that if I get sent to Japan, I’m not coming home,” he said.

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