Japanese health authorities have reported the first two deaths linked to the Covid-19 outbreak on board a Princess Cruises ship under quarantine in Japan.

The two victims, a Japanese man and woman, were described as being in their eighties. The man is said to have had a pre-existing respiratory condition.

The deaths were revealed as Japan comes under increasing criticism for the way in which it handled the quarantining of passengers on the 115,900-gt Diamond Princess (built 2004).

The vessel was put into isolation on arrival in Yokohama on 3 February, after a passenger on its previous voyage tested positive for the virus shortly after disembarking in Hong Kong.

Despite passengers being confined to their cabins, the virus quickly spread throughout the ship. More than 630 passengers and crew have tested positive for the disease, together with several Japanese health officials.

Kentaro Iwata, a respected Japanese disease expert who was allowed on board the Diamond Princess on Tuesday as a member of the government's disaster medical assistance team, described the situation on the Diamond Princess as "completely chaotic" in a YouTube video that has been removed.

Iwata said he felt far more vulnerable on board the ship than at any time when he assisted in previous Ebola and Cholera outbreaks.

"The cruiseship was completely inadequate in terms of infection control," Iwata said.

"There was no distinction between the green zone, which is free of infection, and the red zone, which is potentially contaminated by the virus."

Iwata has since placed himself under a 14-day quarantine to avoid potentially infecting his family.

The Japanese government is vehemently denying that its actions exacerbated the outbreak on the ship.

Chief Cabinet Secretary Yoshihide Suga acknowledged the government had been making unprecedented decisions in dealing with the epidemic and had been doing its best under the circumstances.

"It's a first for us," Suga said at a press conference on Thursday.

Several countries whose citizens were also passengers aboard the Diamond Princess have begun evacuating them out of Japan. They face 14-day quarantine periods on arrival back home.

The US removed 300 of its citizens from the ship earlier this week, flying both virus-free and infected passengers home on chartered cargo planes.

British nationals were due to be evacuated on Friday.