A post-Brexit “hard border” between Northern Ireland and the Republic of Ireland would put police “in the terrorist firing line”, the Northern Ireland Police Federation has warned.
Mark Lindsay, chair of the federation, said he was concerned physical border posts would make officers easy targets.
He added that enforcing border controls would have an adverse impact on PSNI resources and community relations.
The government has repeatedly said it wants to avoid “borders of the past”.
Ministers have suggested that new technological solutions could replace the long traffic queues and customs checks that were once a regular feature of life along the Irish border.
However, Mr Lindsay said he believed the police would have to enforce new post-Brexit regulations “whether its a soft or hard border”.
“No matter who is resourcing them, whether Customs and Excise, the Border Agency, there’s always going to be some element of security threat,” he said.
“That will then become incumbent on the police to provide that protection for those agencies. That puts our officers into the firing line. Their patterns become very predictable.
“It makes them a very predictable target for anybody who wants to attack them from a terrorist point of view.”
Last week, Prime Minister Theresa May said her Brexit plan would mean the UK leaving the European Customs Union in order to negotiate trade deals freely.
Sinn Féin said leaving the customs union would create a “hard border on the island of Ireland” and damage the Good Friday Agreement.
The Northern Ireland Police Federation head said enforcing a hard border would be “a step backwards” for PSNI community policing.
“A lot of communities straddle the border and to start blocking their roads again would come up against real problems,” said Mr Lindsay.
“You would have to have additional resources put down along the actual border itself. We don’t have the resources at the moment to do this.”