Nadia Eweida, 60, took the airline to a tribunal when she was sent home from work for displaying a small silver crucifix on a chain around her neck.
Her claims of religious discrimination were rejected in Britain but yesterday judges in Strasbourg found in her favour.
The European Court of Human Rights (ECHR) ruling concluded there had been a violation of Miss Eweida's right to demonstrate her faith, which caused her "considerable anxiety, frustration and distress".
They rejected similar claims made by another three Christians.
Nurse Shirley Chaplin, marriage counsellor Gary McFarlane and registrar Lillian Ladele lost their cases in the same ruling. They can now appeal against the decision at the Grand Chamber of the Court.
Miss Eweida left her job in airport check-in in September 2006 but returned to work in customer services at Heathrow's Terminal 5 in February 2007, after BA changed its uniform policy on visible items of jewellery.
She said she was "jumping for joy" following the ECHR's decision but expressed disappointed for the other three applicants.
Speaking outside her lawyer's chambers in central London, she said: "I'm very pleased that after all this time the European court has specifically recognised, in paragraph 114 in the judgment, that I have suffered anxiety, frustration and distress."
Prime Minister David Cameron welcomed the ruling on Twitter. He wrote: "Delighted that principle of wearing religious symbols at work has been upheld – people shouldn't suffer discrimination due to religious beliefs."
The British Government was ordered to pay Miss Eweida €2000 (£1600) in damages and €30,000 (£25,000) to cover costs.