An expert on infectious diseases who worked on the Ebola outbreak in Africa said he had never feared as much for his safety as he did aboard the cruise ship locked down by coronavirus.

Kentaro Iwata was ejected from the Diamond Princess after raising questions about quarantine lapses, amid mounting criticism of the government's handling of the crisis on board.

In a damning expose on YouTube, Professor Iwata, a Japanese infectious diseases professor at Kobe University, said he had been asked to leave after expressing concerns about "chaotic" scenes that could spread the Covid-19 virus.

A bus carrying passengers leaves the port after they disembarked from the quarantined Diamond Princess. Photo / AP
A bus carrying passengers leaves the port after they disembarked from the quarantined Diamond Princess. Photo / AP

He claimed passengers were asked to sign consent forms that could have passed on the infection. He is now in self-quarantine, fearing he might have contracted it.

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A further 79 cases were diagnosed on Wednesday, bringing the total infected on the ship to 621, as hundreds of others finally began to disembark at Yokohama Port at the official end of the 14-day isolation period.

Iwata, who previously worked on the Ebola outbreak in Africa, cholera emergencies and the SARS epidemic in China in 2003, said he had never been so "scared" for his safety as he was on the Diamond Princess.

"It turns out the cruise ship was completely inadequate in terms of the infection control. There was no distinction between the green zone, which is free of infection, and the red zone, which is potentially contaminated," he said.

Buses carrying Australian passengers from the quarantined cruise ship leave a port in Yokohama. Photo / AP
Buses carrying Australian passengers from the quarantined cruise ship leave a port in Yokohama. Photo / AP

"There was no single professional infection control person inside the ship. The bureaucrats were in charge of everything."

Iwata later said the disembarkation process was fine but passengers must undergo quarantine in their home countries.

Japanese officials denied Iwata's claims and defended their approach.

Yoshihide Suga, a government spokesman, said: "Since February 5, we have taken thorough measures to prevent the spread of infection."

Passengers who tested negative for Covid-19 will be allowed to depart freely from the ship but concerns remain that some could be asymptomatic carriers - of the 79 newly confirmed cases on Wednesday, authorities said 68 were not showing symptoms.

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A passenger disembarked from the quarantined Diamond Princess cruise ship peeks out of a bus window in Yokohama, near Tokyo. Photo / AP
A passenger disembarked from the quarantined Diamond Princess cruise ship peeks out of a bus window in Yokohama, near Tokyo. Photo / AP

Whether more infections had occurred during quarantine than before it began on February 5, was "possible but not proven," Iwata said.

But citing a "worsening situation on this ship", the UK Foreign Office sources said an evacuation flight from Tokyo for more than 70 British nationals would take place within 48 hours.

Passengers will be screened before boarding and only those who test negative will be admitted, although anyone left behind will receive government support.

Among those likely to be left behind are David Abel and his wife Sally, pensioners celebrating their 50th wedding anniversary, who were diagnosed positive on Tuesday.

The couple, who delivered online video updates from their cabin, were transferred to a hostel until hospital beds become available.

Passengers from the quarantined Diamond Princess cruise ship board a plane at Haneda airport in Tokyo. Photo / AP
Passengers from the quarantined Diamond Princess cruise ship board a plane at Haneda airport in Tokyo. Photo / AP

The prognosis was brighter for Alan Steele, the 58-year-old infected while on honeymoon but who was released from hospital yesterday.

From a luxury hotel, he posted photos of a burger, chips and fizzy drink.

"I'm in heaven," he wrote.

Elaine Spencer, on board with her husband John, told the BBC: "We're still waiting for them to give us a flight, and at the moment we feel like we've been left behind."

Spencer said she was told she would only be able to take hand luggage on the plane later this week.