Immigration lawyers say hundreds of people seeking refugee protection in the UK have been warned they could be removed to other countries in Europe, even though Brexit took away Britain’s power to make such transfers.

At the same time as the UK quit the EU’s single market on January 1, it also left the so-called Dublin Regulation, the legal mechanism that allows EU governments to transfer applicants back to other member states where they had previously been registered.

Ministers have so far negotiated no new bilateral deals to replace the regulations. The UK also lacks access to the “Eurodac” database of migrants’ fingerprints that previously allowed officials to identify which people seeking protection in the UK had previously contacted authorities elsewhere in Europe.

Lawyers and refugee support groups nevertheless said many would-be refugees had received letters telling them the Home Office had evidence they had been present in an EU country before reaching the UK. The Joint Council for the Welfare of Immigrants (JCWI) said “almost everyone” who sought asylum in the UK this year had received such a letter.

One letter seen by the Financial Times said the department was still reviewing evidence and added: “As part of this consideration we may make inquiries with safe countries to verify evidence or ask if, in principle, they would admit you.”

The letter warned the recipient his previous presence in other European countries might render his claim inadmissible.

“If your claim is treated as inadmissible, we will not ask you about your reasons for seeking asylum or make a decision on your protection claim,” the letter said.

The letter gave no clear timescale for the assessment but offered the applicant “help and advice” on returning to his country of origin.

The letter was sent to a non-Arab fleeing Darfur, in Sudan, where the International Criminal Court has alleged non-Arabs have been subject to crimes against humanity, including genocide. Such people are presumed eligible for refugee protection if they reach the UK.

Minnie Rahman, JCWI’s campaigns director, said “almost everyone” who made an asylum claim in the UK since January had been sent such a “threatening letter”.

“These ominous allusions to removal are putting already vulnerable clients through undue stress and trauma, and are patently ridiculous given that no third country has agreed to facilitate such returns,” Rahman said. “The Home Office is simply wasting everyone’s time.”

Nearly 30,000 asylum claims were lodged last year in the UK despite a slowdown created by coronavirus-related travel restrictions. If claims have continued at a similar rate to 2020, this suggests about 10,000 applications will already have been submitted this year and hundreds or even thousands will have received warning letters.

There were 55,133 asylum cases awaiting decisions at the end of 2020, the highest number in the decade since the current counting method was adopted. Home secretary Priti Patel has pointed to the backlog as evidence of the need to handle asylum cases faster, including by accelerating returns of migrants who have passed through safe countries before reaching the UK.

The plans formed a central part of her New Plan for Immigration, announced in March, which seeks to sharply curtail the rights of asylum seekers and is expected to feature in the Queen’s Speech on May 11.

However, there is no legal obligation on asylum seekers to seek protection in the first safe country they reach, and Colin Yeo, a leading immigration barrister, said it was “totally pointless” to delay consideration of applicants’ cases pending a potential removal.

“They cannot be removed to a third country, so they have to be supported in this country for the six months or however long it takes [for the potential removal to be assessed],” Yeo said. “Then their claims will be considered anyway.”

More than half — 52 per cent — of people applying for asylum in the UK in 2019, the last full year for which figures are available, were granted refugee protection either after an initial investigation or on appeal.

The hold-ups were preventing the “genuine refugees” from getting “on their feet”, Yeo said. “It’s also enabling those who don’t have a genuine claim to live here and be supported,” he said. “It’s crazy on that level as well.”

The home secretary has faced persistent criticism for her department’s treatment of people seeking refugee protection, particularly the decision to house hundreds of them in shared dormitories at Napier barracks, a disused army facility near Folkestone where hundreds contracted Covid-19.

The Home Office insisted migrants should make asylum claims in the first safe country they reached rather than making “dangerous and illegal” journeys to the UK. It said rules introduced in December had made claims inadmissible where people had travelled through or had a connection to safe countries.

“Our new plan for immigration will overhaul our asylum system and speed up the removal of failed asylum seekers and dangerous foreign criminals,” the department said.