According to the company’s travel logs, shared with the Press Association, the age of the missing British citizens ranges from 13 to 65.
Alpine Eco Trek said the youngest member of the group is believed to be a 13-year-old girl, according to PA.
Four of the firm’s Nepali tour guides are also missing, PA reported.
The company’s manager Kumar Adhikari said he was in contact with the guides until 08:40 when they were expected to be at Tibet’s immigration office, but their phones were disconnected.
“We were hoping that inside it was safe,” he told PA, “but later the video came from China’s government which [showed] even Tibet’s side was hit by the flood.”
Adhikari said he was “slowly responding to” the families of the Britons but has been unable to give them any updates, adding he has been in contact with the British consulate and forwarded them copies of the clients’ passports.
“I feel so sad. We are just thinking about what to do next…We are in pain at the moment, deep pain.”
Another tour company impacted by the flash floods, Himalayan Glacier US, told the BBC they have lost contact with a group of 47 guests and staff who where making a pilgrimage to Mount Kailash, which included two British citizens.
Foreign Secretary Ed Miliband said he is “deeply concerned” by news of the flash flooding, and wrote on social media: “All my thoughts are with those affected and with British nationals in the area and their relatives here at home”.
In a joint statement with Australia, the EU, Finland, France, Norway and Switzerland, the UK embassy in Nepal urged British nationals in Nepal to “exercise caution in affected areas”.
