Veterans and their families are attending, along with the Prince of Wales, the Duchess of Cornwall and Defence Secretary Des Browne.
The service marks the 68th anniversary of the battle that cost 544 airmen.
It was these crewmen Winston Churchill referred to in his phrase "never was so much owed by so many to so few".
The service is also being attended by Chief of the Air Staff, Air Chief Marshal Sir Glenn Torpy.
After the ceremony, the Prince of Wales, who is patron of the Battle of Britain Fighter Association, and the Duchess were due to meet former aircrew members.
Four Tornado F3s were also taking part in a fly-past over Westminster Abbey.
The defeat of Nazi Germany's Luftwaffe by the RAF's Hurricane and Spitfire pilots came at a cost of more than 1,000 fighter planes.