Shipping asylum seekers overseas for processing risks eroding international protection for refugees, the UN refugee agency’s UK representative has said after reports that Priti Patel will push the proposal as part of immigration reforms.

The home secretary is expected to publish details next week of plans to remove people who arrive in the UK via unofficial means, such as crossing the Channel in small boats, to a third country while claims are considered.

Government sources confirmed the proposals were being looked at but denied reports that specific destinations, including the Isle of Man, Gibraltar and Scottish islands, were on the table.

The broader proposal of offshoring asylum claims, which has echoes of Australia’s highly controversial model, was met with a backlash from migration experts and humanitarian organisations.

Rossella Pagliuchi-Lor, the UK representative for the UNHCR, the UN’s refugee agency, said the organisation was “extremely concerned by these reports”.

She said the UNHCR, which serves as the guardian of the 1951 convention relating to the status of refugees and to its 1967 protocol, a piece of international legislation to which the UK is a party, had not been consulted on any plans to send asylum seekers abroad.

“We urge the UK (and other countries) to refrain from these practices,” she said. “These obligations cannot be outsourced and transferred without effective safeguards in place, both in law and in practice.

“Externalisation arrangements can run counter to the spirit of international solidarity and burden-sharing and risk a gradual erosion of the international protection system, which has withstood the test of time. We all have a collective responsibility – and a common interest – to safeguard that system.”

She added: “We think the solution lies in a better-designed UK asylum system, properly resourced, with simplified procedures where appropriate. That will result in fairer and faster procedures, and lower overall costs.

“It will also limit the possibility of abuse. Returns for those not in need of international protection have to be part of the solution. But what’s often forgotten amid all the recent noise around Channel crossings is that asylum claims in the UK have been falling, and remain far lower here than in countries like France and Germany. The situation in the UK is manageable.”

Boris Johnson insisted the proposals were humane. Answering questions on the proposals during the No 10 Covid briefing on Thursday, the prime minister said: “The objective is a humanitarian one and a humane one, which is to stop the abuse of these people by a bunch of traffickers and gangsters. That’s what we’re trying to do.”

However, Johnson and Patel’s claims that tough measures will deter migrants from making perilous journeys with the help of smugglers are not supported by migration experts, who argue that desperate people fleeing violence and abuse will continue to take risks regardless.

Mike Adamson, the chief executive of the British Red Cross, said: “Offshoring the UK’s asylum system will do nothing to address the reasons people take dangerous journeys in the first place and will almost certainly have grave humanitarian consequences.

“From the children rescued by the Kindertransport to those displaced by the decade-long conflict in Syria, providing sanctuary in the heart of our communities is what’s needed for people fleeing conflict and persecution and should be a key feature of global Britain as a force for good. Being housed in facilities offshore is the opposite of that.”

Sonia Lenegan, the legal director at the Immigration Law Practitioners’ Association, said: “The home secretary says that she wants to stop people smugglers putting people’s lives at risk, but the risk of harm in the government’s proposals is immense. This is not a safe option, and it will be the government putting refugees’ lives at risk instead of people smugglers.

“Offshore processing is also hugely expensive, and we have seen how third countries can use these arrangements to their political advantage, such as when Turkey threatened the EU with opening its borders in February last year.”

Enver Solomon, the chief executive of the Refugee Council, said: “We know from the Australian model that offshore detention leads to appalling outcomes including high levels of self-harm and mental illness. It is an inhumane policy that undermines our nation’s proud tradition of providing protection to people fleeing persecution and terror, many of whom have gone on to work as doctors and nurses in the NHS.”

A series of leaks last year suggested the UK government was considering a number of Australian-style policies, including sending asylum seekers to be processed on Ascension Island, more than 4,000 miles from the UK, which sparked comparisons to Australia’s controversial use of Manus Island and Nauru as offshore detention centres.

A Home Office source said: “Whilst people are dying making perilous journeys we would be irresponsible if we didn’t consider every avenue.” However, the source played down reports that destinations being considered included Turkey, Gibraltar, the Isle of Man or other British islands, saying this was “all speculation”.

The chief minister of Gibraltar, Fabian Picardo, said he had had no discussions with anyone from the UK government and had written to Patel to confirm that the reports were “groundless”, while an Isle of Man source said there were “no foundations” to the reports.

On suggestions that Scottish islands were under consideration, the SNP’s home affairs spokesperson, Stuart McDonald, who sits on the home affairs select committee in Westminster, said: “The people of Scotland and the SNP Scottish government want nothing to do with the cruel and callous policies peddled by the Tory government, and we certainly don’t want our islands being used to carry out such inhumane practices that could breach human rights law.”

Nick Thomas-Symonds, the shadow home secretary, said: “The Tories are lurching from one inhumane, ridiculous proposal to another. Last year they were talking about creating waves in the English Channel to wash boats back and buying ferries and oil rigs to process asylum claims. These absurd ideas show the government has lost control and all sense of compassion.”