The Avro 691 Lancastrian was a British and Canadian passenger and mail transport aircraft of the 1940s and 1950s developed from the Avro Lancaster heavy bomber. The Lancastrian was basically a modified Lancaster bomber without armour or armament and with the gun turrets replaced by streamlined metal fairings, including a new nose section. The initial batch was converted directly from Lancasters; later batches were new builds.

In 1943, Canada's Victory Aircraft converted a Lancaster X bomber for civilian transport duties with Trans-Canada Airlines (TCA). (After the war Victory Aircraft was purchased by what became Avro Canada). This conversion was a success, resulting in eight additional Lancaster Xs being converted. The "specials" were powered by Packard-built Merlin 38 engines and featured a lengthened, streamlined nose and tail cone. Range was increased by two 400 gal (1,818 L) Lancaster long-range fuel tanks fitted as standard in the bomb bay. These Lancastrians were used by TCA on its Montreal–Prestwick route.

The modification of abundant military aircraft into desperately needed civilian transports was common in the United Kingdom in the immediate postwar period; the Handley Page Halton was a similar conversion of the Halifax heavy bomber.

In 1945, deliveries commenced of 30 British-built Lancastrians for BOAC. On a demonstration flight on 23 April 1945, G-AGLF flew 13,500 mi (21,700 km) from England to Auckland, New Zealand in three days, 14 hours at an average speed of 220 mph (354 km/h).

The Lancastrian was fast, had a long range, and was capable of carrying a heavy load, but space inside was very limited as the Lancaster had been designed with space for its seven crew dispersed throughout the fuselage, and with the majority of the load being carried in the 33 ft (10.05 m) long bomb bay. Consequently, as passengers are bulky but low in weight, it was not suited to carry large numbers of passengers, but was suitable for mail and a small number of VIP passengers. BOAC used it for flights between England and Australia from 31 May 1945. It also served with the RAF; RAF Lancaster I serial number PD328, was converted to a Lancastrian and renamed Aries, as well as serving with Qantas and Flota Aérea Mercante Argentina.

15 Lancastrians made over 5,000 trips during the Berlin Airlift to transport petrol. In 1946 a Lancastrian operated by BSAA was the first aircraft to make a scheduled flight from the then-newly opened London Heathrow Airport.

Data from: Avro Aircraft since 1908

With the advent of gas turbine engines there emerged a need to test the new engines in a controlled flight environment in well instrumented installations. An ideal candidate emerged as the Avro Lancastrian which could easily accommodate the test instrumentation as well as fly on the power of two piston engines if required. Several Lancastrians were allocated for engine test-bed work with turbojet engines replacing the outer Merlin engines or test piston engines in the inner nacelles. Fuel arrangements varied but could include kerosene jet fuel in outer wing tanks or fuselage tanks, with avgas carried in remaining fuel tanks.