1. In exceptional circumstances, where the overall functioning of the area without internal border control is put at risk as a result of persistent serious deficiencies relating to external border control as referred to in Article 21 of this Regulation or as a result of the non-compliance of a Member State with a Council decision referred to in Article 19(1) of Regulation (EU) 2016/1624 of the European Parliament and of the Council , and insofar as those circumstances constitute a serious threat to public policy or internal security w ...[+++]ithin the area without internal border control or within parts thereof, border control at internal borders may be reintroduced in accordance with paragraph 2 of this Article for a period of up to six months.

1. In cases where the Commission finds that there are persistent serious deficiencies related to external border control identified in the framework of a rigorous evaluation process, in accordance with Article 15 of Regulation (EU) No ./2012 [on the establishment of an evaluation and monitoring mechanism to verify the application of the Schengen acquis], as a measure of last resort, and insofar as these deficiencies constitute a serious threat to public policy or to internal security within the area without internal border controls, border control at internal borders may be reintroduced for a period of no more than six months.

2. Where no other measures are capable of effectively mitigating the serious threat identified, the Commission shall transmit to the European Parliament and to the Council a recommendation assessing the necessity of the reintroduction of border control at internal borders, as a measure of last resort, to protect common interests within the area without internal border controls.

(5a) Full free movement within the area without internal border controls should be guaranteed and the reintroduction by Member States of border control at internal borders on political grounds should be avoided.

Based on the experience gathered so far with respect to the functioning of the area without internal border controls, the Commission should draw up guidelines on the reintroduction of border control at internal borders, both in cases which require such a measure as a temporary reaction and in cases where immediate action is needed.

The crossing of the external border of a large number of third-country nationals might, in exceptional circumstances, justify the immediate reintroduction of some internal border controls, if such a measure is needed to safeguard public policy and internal security at the Union or national level from a serious and urgent threat.