Senior PSNI officers have warned British MPs that a hard post-Brexit border would be an obvious area of attack for dissident republican terrorists.
Giving evidence to the House of Commons Brexit Committee in Co Armagh, the PSNI's Deputy Chief Constable Drew Harris said any infrastructures along the border would give terror groups "a further rallying call to drive their recruitment."
"They have a focus on this. They see it as an opportunity.
"Infrastructure on the border would be an obvious point for dissident groups to rally around and attack," Mr Harris added.
He said the threat from dissident republicans remains severe and that there were four attempts on the lives of police officers over the past year. There were also 58 shooting and over 32 bombing incidents.
Mr Harris told the committee it was regrettable that a lot of the current border conversations "take us back to the 80s."
"We in law enforcement see no rationale for that infrastructure at the minute," he insisted.
Mr Harris said that during the 1980s there was a major problem with cross-border burglaries of older people in their homes.
He said many of the culprits escaped justice by heading over the border.
The Deputy Chief Constable outlined a number of shared European initiatives, such as the European Arrest Warrant, shared information systems and joint investigation teams, which have helped in the fight against crimes including human trafficking and drug smuggling.
Mr Harris told the committee he would be concerned if the shared systems were lost.
"The systems are for a safer Europe. It is not a one-way street. We have a lot to offer our European partners after Brexit.
"We are unsure of what the landscape is going to look like going forward. Our responsibility is to do our very best to maintain the safety of everyone on this island.
"That has to be backed up with legislation and policy which allows intelligence and evidence to be shared quickly."
When asked what would happen in the event of a no Brexit deal, Mr Harris said the PSNI would have to fall back on existing legal provisions with the Garda in the Republic.
However, he said a new extradition treaty would be needed in the absence of the European Arrest Warrant.
Assistant Chief Constable Stephen Martin, head of the PSNI's serious and organised crime branch, told committee members there is good cross-border cooperation between the PSNI and An Garda Síochána.
He said the two organisations assist each other "across a whole spectrum of crime."
Mr Martin said the PSNI has been sharing intelligence with the gardaí in relation to the ongoing drugs feud in Dublin between the rival Hutch and Kinahan gangs that had resulted in a number of murders.
One of the members of the House of Commons committee that travelled to Co Armagh said "new thinking" is needed on the border issue.
Conservative MP Craig MacKinlay told RTÉ’s Six One News that the European Union is only looking at models of borders that currently exist, but it should consider different, new models that could use technology to resolve customs and tariff difficulties.
Mr MacKinlay said that he was "always aware" the Irish border "is one of the conundrums" of Brexit but said the UK has been put into "something of a corner" by the EU, who have insisted they resolve three key areas ahead of holding trade talks.
He said it is very difficult to decide on the future status of the border "until we can start talking about what future trade relationships will mean."
Mr MacKinlay has said it is not unreasonable to look at the border as part of the "totality" of talks.
He said his way forward would be to have mutual recognition of standards across agriculture and other products with zero tariffs.
He said it is very obvious that the UK has no intention ever of putting up a [physical] border.