Check-in worker Nadia Eweida, 55, of Twickenham, said she was effectively "forced" to take unpaid leave after refusing to conceal the symbol.
BA denied it had banned the wearing of crosses.
It said its uniform policy stated that such items could be worn if concealed underneath the uniform.
It said items such as turbans and bangles could be worn as it was impractical for staff to conceal them.
A former government minister called on Christians to boycott British Airways flights yesterday as the backlash grew over its decision to ban a Heathrow check-in worker from wearing a cross round her neck.
Ann Widdecombe, a former Home Office minister and devout Roman Catholic, said that if BA had not reversed its "crazy" policy by Monday evening, she would cut up her BA executive club card and refuse to fly with the airline.
Urging other Christians to join her in a mass boycott that would inflict huge commercial damage on the business, she said BA was guilty of "persecuting" an employee on the grounds of her faith.
One option would be for Christians to write to the company's chief executive. "But the real power will be their economic power - don't fly BA," she told the BBC programme Heaven and Earth.
Clash of civilizations, anyone?