The British Leyland ADO77 was a prototype built in 1977 as a possible replacement for the Morris Marina.

The Morris Marina was developed and launched in a hurry. This meant that not long after it's release, BL was looking for a replacement. The Marina did perform reasonably well on the market, regularly featuring in the UK’s top-five selling cars. Buyers liked its compact dimensions, large boot and lively performance.

The ADO77 was based on the ADO73, a previously conceived Marina replacement. The ADO77 was going to be considerably larger than the car it replaced. Due to the success of the Ford Cortina MkIII, that had grown in size compared to the MKII showed BL that the market was buoyant and the country’s economy improving so buyers wanted bigger, more powerful cars. The ADO77 was to be fitted with a 2-litre O series engine and have a wheelbase of 100-inches to help nuzzle into the Cortina's market.