Police do not have the numbers to protect a hard Irish border, a representative body has warned.
Planning for a no-deal Brexit needs to begin urgently as it would cost tens of millions of pounds to secure the porous 300-mile (483km) frontier, the Police Federation for Northern Ireland (PFNI) said.
All sides in the discussions between the EU and UK are united in opposition to a heavily-militarised border that characterised arrangements during the long Northern Ireland conflict.
Federation chairman Mark Lindsay said his members would be "sitting ducks" for terrorists if they had to regularly and predictably protect other agencies like customs officers at crossing points from Northern Ireland into the Republic.
He said: "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.
"No-one wants a 'hard' border, but if there 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 frontier between Northern Ireland and the Republic 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 (PSNI) 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.
"Contingency planning has to begin as a matter of urgency."
The Federation has been calling for an increase in the number of officers for years.
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 he required.
In October the PSNI launched a new recruitment drive and said they were hoping to sign up 300 new officers.
Next year some 730 are eligible to retire - 11% of the workforce. A further 331 can retire over the following two years, the Federation disclosed.