Thirty-three people from the UK, including a 13-year-old girl and 14-year-old boy, are among hundreds feared missing following a deadly flash flood near the Nepal-Tibet border.
At least 165 people have died after flooding and mudslides on Wednesday, Nepal's National Disaster Risk Reduction and Management Authority (NDRRMA) has said.
It added that 826 people are missing, with the number of missing tourists said to be more than 529.
Prime Minister Andy Burnham said he is in touch with local authorities and "will do everything we can" to support British nationals, urging those in the region "to follow guidance from the local authorities and follow our travel advice".
The UK Foreign Office said it is working closely with Nepalese authorities "following concerning reports of British nationals who are missing" and has encouraged Brits to follow local guidance and "remain alert to the changing situation".
According to police, 113 of the missing tourists are Nepalese, and 54 are from the US.
Excursion company Alpine Eco Trek, which offers guided expeditions and tours in Nepal, confirmed to the BBC that it has not been able to establish contact with a group of 12 Britons who were booked on a tour to Mount Kailash, and were in the region when the flash floods struck.
According to the company's travel logs, shared with the Press Association, the age of the missing British citizens ranges from 13 to 65.
It said the youngest member of the group was a 13-year-old girl.
The company said it had made attempts to charter a rescue helicopter but were unable to fly due to poor conditions, and will continue to help with ongoing rescue efforts on Thursday.
It also reported four of the firm's Nepali tour guides are also missing.
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."
Other tour companies have also been impacted by the flash floods, with British citizens among other tours being affected.
The Trekkers' Society, which organises tours in Nepal, Tibet, and Bhutan, said nine UK nationals on a pilgrimage to Mount Kailash are missing after they crossed the Nepal-Tibet border.
It said four males, aged between 14 and 50, and five females aged 40 - 53 remained unaccounted for.
The society's general manager said the group were at Chinese immigration at the border when the flood happened, but it has no contact with the tour guide since.
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.
Kailash Journeys also reported a group of 27 guests and staff missing, with a spokesperson saying they were last in contact with them early Wednesday morning local time.
A list of names provided to the BBC included two men who are 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".
The flood hit early on Wednesday in Rasuwa, just north of Nepal's capital Kathmandu, triggering a landslide.
Preliminary information suggested it was caused by an earthquake, but the US Geological Survey said additional analysis indicated "the seismic energy was instead generated by a landslide."
It said a magnitude 5.2 seismic event caused by a "glacial collapse and debris flow" occurred near the border at 08:37 local time on Wednesday.
Dramatic CCTV footage showed a wall of water and mud hitting buildings near a border crossing and video filmed from inside a bus in Trishuli captured dozens of people trying to flee a flood surge that was surging through the valley.
Meanwhile Chinese state media outlet CCTV said a mudslide in Tibet's Gyirong killed three people and left 265 missing, with rescue efforts under way.
