The Home Office is preparing to end the current system of family reunification for asylum-seeking children if the UK leaves the EU without a deal, the Guardian has learned.

The government has privately briefed the UN refugee agency UNHCR and other NGOs that open cases may be able to progress, but a no-deal Brexit would mean no new applications after 1 November from asylum-seeking children to be reunited with relatives living in the UK. Even if there is a deal, the future of family reunion is not certain.

Lawyers and campaigners say they will be trying to get through as many claims as possible in the next two months, warning that the impact on migrant children stranded alone in countries such as Greece and Italy could be “fatal” as more head for the Channel to try to cross to the UK irregularly.

A spokesman for the UNHCR said: “[We understand] that if the UK leaves the EU without a deal, the Dublin Regulation, which allows for the transfer of asylum-seeking children and adults within the EU to join family members, will no longer apply to the UK.

“UNHCR urges the UK government and its European partners to work together to ensure that appropriate arrangements remain in place for asylum seekers, refugees and stateless people.”

The Home Office has previously been criticised for making it difficult for young migrants to join family in the UK but lawyers say it is still a vital route for highly vulnerable child migrants, many of whom live on the streets in mainland Europe.

Efi Stathopoulou, the project coordinator at Refugee Legal Support in Athens, said the family reunification route was the only way she could persuade vulnerable young people to engage with the authorities.

“Children come here very afraid,” she said. “There have been cases where it was very obvious they were being exploited. Without the possibility of a safe way to reach the UK, these young people will simply vanish to try to cross the Channel at Calais on lorries or boats.”

Stathopoulou said Greek authorities asked her earlier this year to help speed up claims if a no-deal Brexit looked likely in order to get as many children as possible processed before the deadline.

Family reunification is vital because of widespread homelessness and exploitation faced by migrants in Greece.

“We just helped a boy reach the UK who was street homeless in Athens despite already being being highly vulnerable,” said Stathopoulou. “He had lost his entire [immediate] family to a bomb in Afghanistan. Another boy was raped because he had nowhere safe to sleep.

Facebook Twitter Pinterest Efi Stathopoulou, the coordinator of Refugee Legal Support based at the Athens Solidarity Centre. Photograph: Giorgos Moutafis/The Guardian

“I’m here when they make calls to their relatives in the UK; it’s so hard to see, there is so much emotion, their aunts and uncles just want them to be together.”

The Home Office confirmed that once the UK left the EU, the only commitment would be to cases not resolved by that date.

A spokesperson said: “Deal or no deal, cooperation will continue on asylum and returns as it is in the interests of the UK and the EU. That is why we have taken proactive action to ensure that whatever the circumstances, ‘Dublin’ requests relating to family reunification that have not been resolved on the date we leave will continue to be considered under existing rules.”

There has been a huge surge in sea arrivals to Greece in the past month, putting pressure on already overcrowded and dangerous reception centres on the Greek islands.

In the first six months of 2019 alone, there were nearly 700 requests through Greek authorities to the UK to take charge of an asylum seeker under the “Dublin” asylum law. Since 2013, there have been 2,450 such requests.

The only other legal route for any asylum-seeking child to reach the UK from within Europe is the Dubs scheme, named after Lord Alf Dubs who pushed it through parliament.

MPs and campaigners hoped the Dubs scheme would resettle about 3,000 children but ministers controversially set a limit of 480. About 270 children have so far come, according to charity Safe Passage, which has lobbied for more safe routes.

Beth Gardiner-Smith, the chief executive of Safe Passage International, said: “If the government fails to protect family reunification, the consequences could be fatal. Too often children risk their lives on lorries and dinghies because the legal process is taking too long. If they lose that right altogether, these dangerous journeys will only increase.

“We know that if the political will is there, swift and safe transfers are possible; a no-deal Brexit does not mean transfers must stop.”

If the UK leaves with a deal similar to that negotiated by Theresa May, the future of asylum cooperation will be renegotiated as part of the transition period.

Claude Moraes, a Labour MEP, said he would be raising questions with the Home Office about the plans and believed that the department would have to share responsibility if it wanted Europe to take back asylum seekers from the UK.

He said: “If [home secretary] Priti Patel wants to send people back to Europe, then the UK will have to think about compassionate responsibility sharing for vulnerable child migrants. In this sense, the UK is not an island.”

