Senior PSNI officers have warned MPs that a hard Irish border would be an obvious target for dissident republican terrorists.
Deputy Chief Constable Drew Harris warned it would give terrorist groups "a further rallying call to drive their recruitment".
DCC Harris was giving evidence before the House of Commons Brexit Committee who were on a visit to Co Armagh, to gauge local opinion on the challenges and opportunities presented by leaving the EU.
"Infrastructure on the border would be an obvious point for dissident groups to rally around and attack," he said.
The threat from dissident republican groups remains severe, he said, with four attempts on the lives of PSNI officers in the past year alone.
In this time, there were also 58 shootings 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 rational of 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 the Garda.
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 Garda in relation to an ongoing drugs feud in Dublin between the rival Hutch and Kinahan gangs that had resulted in a number of murders.