Drones will monitor migrant boats attempting to reach the south east of England, pilots have been warned.
Remotely-piloted aircraft will be searching a large area of the Channel, from Eastbourne to Margate.
The flights are for "the purpose of national security and protection of human life", the Civil Aviation Authority (CAA) said.
The Home Office said it was working to identify the "most effective" ways to "tackle small boat crossings".
More than 1,700 people have crossed the Channel since the government declared a "major incident" on 28 December 2018.
The airspace restrictions came into force on 2 December and last until 31 March next year.
The CAA said aircraft can only enter the "temporary danger areas", which can be put in place with less than 24 hours' notice, after being cleared by air traffic control.
They are limited to 1,200ft above sea level and will not affect commercial passenger planes, the CAA said.
The Home Office declined to say who would be operating and providing the drones, which will fly from Lydd Airport.
It said the government was working with "partner agencies" to "make use of available assets".
In June, former immigration minister Caroline Nokes told Parliament the UK had "recently delivered drones and other surveillance equipment to France, enabling their law enforcement officers to intercept and disrupt attempted crossings".