Russian hoaxers who apparently tricked Prince Harry into offering help to take penguins to the North Pole have raised serious questions over security and screening measures for the Duke and Duchess of Sussex as they leave the royal fold, a royal expert said.

Pretending to be putting through the Swedish activist Greta Thunberg and her father, Svante, hoaxers Vladimir Kuznetsov and Alexey Stolyarov managed to reach Harry on his landline at his rented Vancouver Island mansion on New Year’s Eve and on 22 January, it has been reported.

The royal, seemingly duped into thinking he was talking to the Thunbergs, also criticised Donald Trump and spoke of a “bullying” tabloid media trying to “sink” him and wife Meghan.

A spokeswoman for the Sussexes declined to comment when asked if there was any doubt the voice was that of Harry.

A former press secretary to the Queen, Dickie Arbiter, claimed the fact that the hoaxers, known as Vovan and Lexus, had reached Harry exposed weaknesses in their personal security. “As long as Harry and Meghan are over there, they’re out of the protection of the system,” he said. “For all its faults, the system does, and is there to, protect.”

He said the hoaxers would not have been able to get through the Buckingham Palace switchboard. “They’re pretty vigilant,” he said, adding: “If you’re outside the system, you’re open to anything and everything.”

The couple has a 15-strong team of staff based at Buckingham Palace, but they will be disbanded when the couple transition on 31 March, with some staff being made redundant and others redeployed in other royal households. No details about any staff in Canada have been made public.

Arbiter spoke as the Sun, which published excerpts of the conversations, reported more details of the hoax calls. Harry failed to spot he was being pranked when the fake Greta and her father said they had 50 penguins that were stuck in land-locked Belarus and they were after a ship to transport them to the north pole, even though the animals are native to the south pole.

When asked if he had any contacts to help, the duke is said to have suggested: “I’ve got one person who is a polar guide in the north pole … he may be able to help you, he knows all the right people.”

“Greta” also asked if Harry could help her marry into the royal family and suggested she was interested in Prince George, the Sun reported. It said Harry replied: “I can assure you, marrying a prince or princess is not all it’s made up to be.”

When the hoaxers suggested there were discussions in Russia that Harry could become head of a restored monarchy, he replied chuckling: “Well there you go, maybe that’s our new purpose: to be able to take over Russia.”

The hoaxers joked about Harry smoking “weed” with hippies on Thunberg’s eco-catamaran, and also of forming a celebrity movement called “Stars Save the Earth” with Leonardo DiCaprio and Angelina Jolie.

During one call they tricked him into believing mining companies close to Trump were exploiting the fictional island of Chunga-Changa – the name of a Russian children’s song.

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The rights to the audio recordings had been “transferred” to British media, the hoaxers said as they confirmed the Sun’s report in response to a Guardian inquiry.

In the audio, a person, reportedly Harry, says of the decision to stand down as a senior royal: “Sometimes the right decision isn’t always the easy one. And this decision certainly wasn’t the easy one, but it was the right decision for our family, the right decision to be able to protect my son. And I think there’s a hell of a lot of people around the world that can identify and respect us for putting our family first.”

On Trump, he says: “I think the mere fact that Donald Trump is pushing the coal industry so big in America, he has blood on his hands.” He says he is confident “things will change” on the climate agenda within 10 years: “But we can’t wait five to 10 years, so I think if Donald Trump can become president of the United States of America, then anything’s possible, right?”

He continues: “You forget, I was in the military for 10 years so I’m more normal than my family would like to believe … But certainly, being in a different position now gives us the ability to say things and do things that we might not have been able to do.”

On Prince Andrew, who has stepped down from public duties over his friendship with the convicted sex offender Jeffrey Epstein, he says: “I have very little to say on that. But whatever he has done or hasn’t done, is completely separate from me and my wife.”

Harry speaks of Boris Johnson being a “good man”, and tells the person posing as Thunberg: “So you are one of the few people who can reach into his soul and get him to feel and believe in you. But you have to understand that because he has been around for so long like all of these other people, they are already set in their ways.”

In separate quotes, published by Mail Online, Harry reportedly says he has been “part of a family and part of a country that is scared of the tabloid media because they have so much power and influence and no morals.

“From the moment that I found a wife that was strong enough to be able to stand up for what we believe in together, [that] has basically scared them so much that they’ve now come out incredibly angry, they’ve come out fighting, and all they will try and do now is try and destroy our reputation and try and, you know, sink us.”

He adds: “It hasn’t been very nice. It’s been horrible, but we will come out of it stronger people.”

Kuznetsov and Stolyarov have previously targeted Elton John, the Turkish president, Recep Tayyip Erdoğan, and the US senator and Democratic presidential hopeful Bernie Sanders.