The plane is currently the most tracked in the world on Flightradar24

The Boeing WC-135R Constant Phoenix, which is a military adaption of the Boeing 707, left from RAF Mildenhall at 07:21 this morning.

The plane flew out to Norway, turning just before Trondheim, before heading back to the UK.

As it approached the East Anglian coast, the plane squawked 7700, meaning to declare a mid-air emergency.

The plane has since landed back at RAF Mildenhall after doing a number of loops.

A number of Typhoon fighter jets have been circling the area, although this is unknown whether they are related to the incident.

It is not known yet the reason for declaring a mid-air emergency, although these kind of incidents are fairly common and could be anything from a technical issue to a sick member of personnel.

The Boeing WC-135R Constant Phoenix is known informally as the 'nuke sniffer' because of its use to collect samples from the atmosphere for the purpose of detecting and identifying nuclear explosions. 

The plane was the most tracked in the world on Flightradar24 at the time of flying.

You can follow it here.

Updates to follow.