Weaker immigration checks in the Irish Republic are providing a route for international criminals to enter the UK, Northern Ireland's police chief has told MPs.
George Hamilton said controls in the Republic of Ireland did not have the same "resource or focus" as those operating in the North.
The Police Service of Northern Ireland (PSNI) chief constable was being questioned by members of a Westminster committee as part of its inquiry into the future of the Irish border post-Brexit.
Mr Hamilton told the Northern Ireland Affairs Committee that a growing number of criminals operating on the island of Ireland were from abroad.
That prompted a question from committee chairman Laurence Robertson on how international gangsters were entering the UK and Ireland.
"How are these people able to access the countries?" he asked. "What is going wrong with border control?"
Mr Hamilton replied: "Access into the Republic of Ireland may not have the resource assigned to it or the immigration checks we would have in Northern Ireland or indeed more broadly into the United Kingdom."
He highlighted that 775 people had been detained at Northern Ireland ports in the last year.
He continued: "I think it would be fair to say that immigration controls into the Republic of Ireland may not have the same resource or focus that we would be seeing in Northern Ireland."
Mr Robertson said his claim was of "wider concern than just over Brexit". He noted the committee had examined the possibility of developing common UK and Ireland travel visas post-Brexit.
"We did so on the basis that checks in Ireland were of the same level and same quality," he added.
"Are you saying that's not the case?"
Mr Hamilton said the PSNI and Irish Garda were "jointly managing" risks around extremism and "radicalised" individuals returning from war-torn countries like Syria.
He added: "We would be seeking to maximise the security right across the island of Ireland in terms of immigration and access into these islands - north, south and into the UK."
On the threat posed by dissident republicans, the chief constable said if police checkpoints were set up on the Irish border in the wake of Brexit they would provide a "static and obvious" target for the violent extremists.
However, he went on to express doubt whether such checkpoints would be staffed by police, given their function was more likely to relate to customs matters.
Mr Hamilton said regardless of the shape of the Brexit deal, the Irish border would always remain "porous", given its size.
He said his priority was ensuring that policing arrangements, such as the European Arrest Warrant, and cross border information sharing facilities were maintained, perhaps via a new bilateral treaty, when the UK leaves the EU.