Planning for a no-deal Brexit needs to begin urgently, as it would cost tens of millions of pounds to secure the porous 483km frontier, says the Police Federation for Northern Ireland.
All sides in the discussions between the EU and UK are united in opposition to a heavily militarised border.
Federation chairman Mark Lindsay said his members would be “sitting ducks” for terrorists if they had to regularly and predictably protect other agencies such as customs officers at crossing points from the North into the Republic.
“Numbers are already painfully thin on the ground and, if hundreds were required along a porous border, the situation would inevitably become intolerable and unmanageable,” he said.
“No one wants a ‘hard’ border, but if the politicians fail to reach an accord or compromise, it seems something that is a lot less relaxed than what we currently enjoy will be the consequence.”
The meandering border encompasses largely remote areas and contains around 275 crossing points.
Mr Lindsay claimed there would have to be a very significant increase in the number of officers in the Police Service of Northern Ireland to secure it.
“Imagine, therefore, what would happen in communities all over Northern Ireland if officers were to be transferred to do border security duties alongside other agencies such as HM Customs & Excise? Cities and towns would see officer numbers decimated,” he said.
“Contingency planning has to begin as a matter of urgency.”
Mr Lindsay said the present below-par total of fewer than 6,700 was already a few hundred below what Chief Constable George Hamilton said was required.
Mr Lindsay said recruitment was unable to keep pace with the numbers leaving, and those figures do not include officers departing for alternative, less stressful employment.
“It’s worth recalling that before the mid-90s [during the Troubles], the border was secured with double the current number of officers, backed by considerable military support,” he said.
The terrorist threat level posed by dissident republicans in the North is rated as severe.