A Royal Air Force Atlas has supported Border Force operations in the English Channel.
The Ministry of Defence (MOD) said it was the first military aircraft to fly in support of Border Force, although the Royal Navy has previously offered assistance.
More than 4,000 migrants have reached the UK in 2020 by crossing the English Channel in small boats.
The fine weather and calm seas of recent days has seen hundreds take to the water in an attempt to reach the UK.
The A400M aircraft left Oxfordshire's RAF Brize Norton on Monday morning to monitor developments in the waters between the UK and France.
According to the MOD, it was deployed for surveillance and intelligence, feeding information directly to the UK Coastguard and Border Force, to allow for the quicker interception of boats.
The aircraft is usually used for transporting personnel and hardware, the RAF said.
The MOD last week received a formal request from the Home Office to support UK Border Force in the Dover Straits, where migrants have been attempting to enter the UK.
The Chancellor Rishi Sunak recently declined to rule out using the Royal Navy to patrol for migrants crossing the English Channel.
Meanwhile, a former Royal Marine has been appointed to lead the UK's response to tackling illegal small boat crossings across the Channel.
Cover image: The aircraft flies over Dover while supporting Border Force operations in the English Channel (Picture: MOD).