Volunteer teachers Katie Gee and Kirstie Trup, both 18, are understood to have been flown into RAF Northolt in London, to be reunited with their families.
Five men are being questioned by police on the Indian Ocean island after the women were attacked by men on a motorbike as they walked along a road on Wednesday night.
Mkadam Khamis, regional police commissioner on Zanzibar, told The Daily Telegraph yesterday: “There are five people we have, all men, who we are interrogating over this matter this morning. They have not yet been arrested. They are co-operating and answering our questions.
“Later today maybe there will be a development legally.”
The young women, from north London, were enjoying the last week of a trip as volunteer teachers to the predominantly Muslim island when a corrosive substance was thrown at them in an apparently unprovoked attack.
They suffered burns to their faces, hands, legs, backs, and necks, The Times reported.
They were flown to the Tanzanian capital Dar es Salaam for treatment before being released.
Marc Trup, Ms Trup’s father, told the newspaper the pair were “inconsolable” and still in pain when he spoke to them on a mobile phone lent to the girls by a passer-by after the attack.
“We couldn’t get anything out of them because they had been burned,” he said. “Both girls are very shocked and very frightened.”
Speaking about his daughter, he added: “She can still see and she is not dead. Whatever it is we will cope with it.”
i-to-i Travel, which organised the girls’ trip, confirmed yesterday that they had arrived back in the UK and been taken to a hospital in the capital.
In a statement outside Chelsea and Westminster Hospital, Mr Andy Williams, consultant burns and plastic surgeon, said: ``We can confirm that Katie and Kirstie have been transferred to our care at Chelsea and Westminster Hospital's burns unit where we're still assessing their injuries.
“Both girls are well and their families are with them. They will be staying at Chelsea and Westminster Hospital.
“Both families would like to thank everyone that’s helped to bring the girls back. The families now wish to have time with the girls and that the media would respect their privacy at this difficult time.”
A family friend, speaking on behalf of the girls’ mothers, Rochelle Trup and Nicky Gee, said: “Both families are extremely upset and distressed at this completely unprovoked attack on their lovely daughters, who had only gone to Zanzibar with good intention.”
Mr Trup said the girls were dressed appropriately and had been warned not to wear anything which gave away their Jewish background, including the Star of David.
He said: “We know it’s a Muslim country, they were Western girls. Unfortunately they went out during the month of Ramadan.
“There has been a huge alert in African countries with potential threats. Maybe it’s connected, maybe not.”
The girls had originally planned to return in time to collect their A-Level results next week, with Ms Trup hoping to study history at the University of Bristol while Ms Gee is considering the University of Leeds, The Times reported.
Their flight home came as news emerged that Ms Gee had been attacked previously on the island.
A friend, Oli Cohen, 21, told The Daily Telegraph: “Katie was attacked two weeks ago by a Muslim woman for singing during Ramadan.
Last night Katie Gee thanked supporters for their good wishes.
Writing on Twitter she said: “Thank you for all your support x”