The flight took off from Geneva ay 7.30am local time and was originally due to land at 8.30am (GMT).
However, the airbus has announced an issue in the air.
It is not yet clear what the emergency involves.
The British Airways plane, which first flew in 2001, flies frequently between the United Kingdom and the European Union.
In the last week it has flown to Oslo, Frankfurt, Paris, Hamburg, Milan and Zagreb.
Flight BA723 has declared a mid-air emergency
The aircraft landed safely after our highly-trained pilots asked for a priority landing as a precaution
The flight landed in Heathrow Airport at 8.09am.
A spokeswoman for British Airways said: “The aircraft landed safely after our highly-trained pilots asked for a priority landing as a precaution.
“Safety and security are always our top priority.”
— Brendan Grainger (@S118869) March 9, 2017
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Heathrow Airport has been contacted for comment.