The injuries sustained by Kirstie Trup and Katie Gee after being attacked with acid are “absolutely horrendous” and “beyond imagination”, said Katie’s father Jeremy Gee.

Mr Gee told the Daily Mail: “The photographs I have seen are absolutely horrendous.” He said: “The level of the burns are beyond imagination.”

The two Jewish 18-year-olds from north-west London were being treated in a central London hospital after arriving back in the UK on Friday afternoon.

They had been attacked in Zanzibar, east Africa, on Wednesday evening by two men on a moped and suffered burns to their hands, chests and faces.

Five men are being questioned by local police concerning the attacks and a warrant has been issued for the arrest of an Islamist preacher, Sheikh Issa Ponda Issal. It is suggested that his teachings could have inspired the attack.

The girls, who were both members of FZY, were in their last week of a volunteering programme arranged by company i-to-i. Both girls attended JFS.

Kirstie’s father Marc Trup told The Times that the girls were dressed appropriately and had been warned not to wear anything that identified them as Jewish , including the Star of David .

According to local deputy police commissioner Mkadam Khamis, "the motive for the attack on the volunteers, aged 18 years, has not been established. Police in Zanzibar have launched a manhunt, and we ask for public assistance in identifying the attackers".

It has been reported that police have offered a £4,000 reward for information on the attackers

There is evidence that there had been more than one previous incident between the girls and locals.

Close friend Oli Cohen, 21, told reporters: "Katie was attacked two weeks ago by a Muslim woman for singing on Ramadan. She was shocked as it just came from out of the blue - but she wasn't scared enough to come home she stayed out there to finish her trip and volunteering.”