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 today judges in Strasbourg found in her favour.
A 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 happy and very happy and very pleased that Christian rights have been vindicated in the UK and Europe.
“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.
“I’m disappointed on behalf of the other three applicants but I fully support them in their asking for a referral for their cases to be heard in the Grand Chamber, and I wish them every success in the future to win.
“I was very selfish initially when I heard the verdict because I was jumping for joy and saying ‘thank you Jesus’.”
Miss Eweida, from Twickenham, south-west London, added: “It’s a vindication that Christians have a right to express their faith on par with other colleagues at work visibly and not be ashamed of their faith.”
The lengthy judgment found a fair balance was not struck between Miss Eweida’s desire to demonstrate her religious belief and BA’s wish to “project a certain corporate image”.
It found the airline’s aim was “undoubtedly legitimate” but said domestic courts accorded it “too much weight”.
It concluded: “Ms Eweida’s cross was discreet and cannot have detracted from her professional appearance.
“There was no evidence that the wearing of other, previously authorised, items of religious clothing, such as turbans and hijabs, by other employees had any negative impact on British Airways’ brand or image.
“Moreover, the fact that the company was able to amend the uniform code to allow for the visible wearing of religious symbolic jewellery demonstrates that the earlier prohibition was not of crucial importance.
“The court therefore concludes that, in these circumstances where there is no evidence of any real encroachment on the interests of others, the domestic authorities failed sufficiently to protect the first applicant’s right to manifest her religion.”
In their ruling, judges rejected claims made by lawyers for the Government who argued Miss Eweida’s rights were only protected in private.
Prime Minister David Cameron welcomed the ruling on Twitter.
He wrote: “Delighted that principle of wearing religious symbols at work has been upheld - ppl shouldn’t suffer discrimination due to religious beliefs.”
The British Government was ordered to pay Miss Eweida 2,000 euro (£1,600) in damages and 30,000 euro (£25,000) to cover costs.
Andrea Williams, director of the Christian Legal Centre, said: “We are delighted that the cross has been recognised and indeed that Nadia has won her case.
“We are saddened that Shirley Chaplin lost her case.
“The court said that although they recognised that the cross was a Christian symbol, the hospital, the Government was entitled to say, that on the grounds of health and safety she should have to take off her cross.
“We have to remember that nurse Chaplin had nursed for 30 years without incident wearing that cross.
“She sought to make every effort to accommodate the hospital’s concerns and they basically wouldn’t shift, they wouldn’t move, they wouldn’t compromise.
“It’s very sad that in a sense what the court did say was that the cross is a Christian symbol but now we’re going to wash our hands of what happens next because the Government, the employer, is allowed to do what it wants.
“What I’m also pleased about is that in the European court they recognised that saying that marriage is between a man and a woman, that believing that sex should be within marriage is actually part of the Christian faith, that flows from believing in the Bible.
“But understanding that is a good step forward but again what the European court did was slightly wash their hands of it at that moment.
“They went on to say that if an employer has an equalities policy and says there should be no discrimination in any way on the grounds of sexual orientation no matter what your Christian belief is that the sexual orientation rights win.”
The ECHR judges rejected Mrs Chaplin’s case on the grounds that the removal of her necklace was deemed necessary to protect the health and safety of nurses and patients.
The nurse, from Exeter, was switched to a desk job after she refused to take off a crucifix which hung round her neck. She took the Royal Devon and Exeter NHS Trust Hospital to an employment tribunal in 2010, claiming that taking off the cross would violate her faith.
A panel later found against Mrs Chaplin, 57, and its decision was upheld by the ECHR where judges concluded that hospital managers were better placed to make decisions about clinical safety than courts.
Miss Ladele, a registrar at Islington Council in north London and Mr McFarlane, a 51-year-old solicitor and father of two from Hanham, in Bristol, also saw their claims rejected.
Judges found Islington Council’s action was “legitimate” given it was also obliged to consider the rights of same-sex couples while they said Mr McFarlane took on the role at counselling service Relate in the knowledge that clients could not be divided up in accordance with their sexual orientation.
They decided Relate’s decision to dismiss the former church elder was designed to enable the organisation to provide a service without discrimination.
Campaigners have been eagerly awaiting today’s judgment which is expected to help shape the future of equality law in the UK.
But a spokesman for the Department for Culture Media and Sport said the Government was unable to comment on how the ruling could effect personal freedom at this stage.
Equalities minister Maria Miller said: “We are delighted the principle that one can wear religious symbols at work has been upheld. People shouldn’t suffer discrimination because of their religious beliefs.
“We are absolutely committed to the rights of Christians and people of all beliefs to follow their faith openly and being able to do so at work is a vital freedom.”
British Airways said the airline was not a party to the legal action which was pursued against the UK government.
A spokesman said: “Our own uniform policy was changed in 2007 to allow Miss Eweida and others to wear symbols of faith and she and other employees have been working under these arrangements for the last six years.
“Miss Eweida has worked continuously for British Airways for 13 years.”