The UK will not accept the return of asylum seekers from Ireland, the prime minister has told MPs.
The Irish government has complained of a significant spike in the number of asylum seekers arriving in Ireland from Northern Ireland, and argued the UK has a duty to take them back.
But Rishi Sunak told the Commons the UK had no legal obligation to do so.
He also urged the Irish government to honour its pledges to avoid a hard border with Northern Ireland.
On Tuesday, Irish Justice Minister Helen McEntee announced she was releasing 100 police officers from desktop duties to work on frontline immigration enforcement, in response to a rising number of asylum seekers who, she claimed, were coming from Northern Ireland because they feared being sent to Rwanda.
During Prime Minister’s Questions, Mr Sunak said Northern Ireland Secretary Chris Heaton-Harris was “seeking urgent clarification” from Dublin that “there will be no disruption or police checkpoints at or near the border”.
“I can confirm that the United Kingdom has no legal obligation to accept returns of illegal migrants from Ireland,” he added.
Downing Street has repeatedly argued that the UK is under no obligation to take asylum seekers back from Ireland, and will not do so while France continues to refuse to accept returns from the UK.
Following Brexit, the 310-mile frontier between Northern Ireland and the Irish Republic is the only land border between the UK and the EU.
But both governments have agreed that a return to security posts here could jeopardise the Good Friday Agreement, which helped to end nearly 30 years of violence in Northern Ireland.
In the Commons, Democratic Unionist MP Carla Lockhart dismissed Dublin’s claim that 80% of asylum seekers were entering Ireland from Northern Ireland as “unsubstantiated”, suggesting “the reverse is true”.
She also accused the Irish government of “hypocrisy” given its previous statements about the border.
Mr Sunak said the UK had “made commitments to avoid a hard border on the island of Ireland” and the “Irish government must uphold its promises too”.
“We can’t have cherry-picking of important international agreements,” he added.
The prime minister said it was “no surprise that our robust approach to illegal migration is providing a deterrent, but the answer is not sending police to villages in Donegal”.
“It’s to work with us in partnership to strengthen our external borders all around the Common Travel Area [an open borders area including the UK, Ireland, the Isle of Man and the Channel Islands] that we share.”
The UK has an operational agreement with Ireland on the Common Travel Area which Dublin says provides for returning asylum seekers, but No 10 has said this is not legally binding and that no one has been returned to the UK under its terms.
Labour said it agreed the UK should not accept returns from Ireland “while Britain is not able to return people who arrive here from the EU”.
But Irish Prime Minister Simon Harris has said his country will not “provide a loophole” for others’ migration “challenges”.
An Irish police spokesman has told the BBC the 100 officers being redeployed will not be assigned “to physically police the border with Northern Ireland”.
The Safety of Rwanda Act, which became law last week, aims to deter migrants from crossing the Channel in small boats by deporting some asylum seekers to the east African country.
The Home Office has announced that migrants identified for deportation to Rwanda have begun being detained.
It said more operations would be carried out in the coming weeks. (BBC)