ICAO Doc 4444:
6.5.6.1.1 The approach sequence shall be established in a manner which will facilitate arrival of the maximum number of aircraft with the least average delay. Priority shall be given to:
a) an aircraft which anticipates being compelled to land because of factors affecting the safe operation of the aircraft (engine failure, shortage of fuel, etc.);
b) hospital aircraft or aircraft carrying any sick or seriously injured person requiring urgent medical attention;
c) aircraft engaged in search and rescue operations; and
d) other aircraft as may be determined by the appropriate authority.
Note.— An aircraft which has encountered an emergency is handled as outlined in Chapter 15, Section 15.1
I recommend reading Chapter 15, it's an interesting read.