The Archbishop of York says British Airways' refusal to let a staff member openly wear a cross is "nonsense".
Dr Sentamu says wearing a cross is important
Dr John Sentamu, the country's second most senior Church of England cleric, urged BA to reconsider its decision.
He said the ruling - allowing male Sikh staff to wear turbans and female Muslim staff to wear hijabs, as they cannot be worn under a uniform - was "flawed".
Heathrow check-in worker Nadia Eweida, 55, of Twickenham, London, lost her appeal against BA's decision on Monday.
She has been on unpaid leave since her bosses first said she could not visibly wear her cross.
BA denied it had banned the wearing of crosses and said Ms Eweida had a right to a second appeal.
It said its uniform policy stated that such items could be worn if concealed underneath the uniform.
But Dr John Sentamu said BA, as a national carrier, ought to consider the place of Christian values represented by the cross.
"For me, the cross is important because it reminds me that God keeps his promises," the Archbishop said.
"Wearing a cross carries with it not only a symbol of our hopes but also a responsibility to act and to live as Christians.
"This symbol does not point only upwards but also outwards, it reminds us of our duties not only to God but also to one another.
"British Airways needs to look again at this decision and to look at the history of the country it represents, whose culture, laws, heritage and tradition owes so much to the very same symbol it would ban."
Expression of faith
Ms Eweida said she was effectively "forced" to take unpaid leave after refusing to conceal the symbol.
After Monday's decision, she said: "I am fairly disappointed but I'm looking forward to the next stage because the cross is important and the truth will be revealed.
"It is important to wear it to express my faith so that other people will know that Jesus loves them."
Ms Eweida said she wanted to be able to do as people of other faiths did and be allowed to wear visible religious symbols.
BA said in a statement: "British Airways has 34,000 uniformed staff, all of whom know they must abide by our uniform policy.
"The policy does not ban staff from wearing a cross. It lays down that personal items of jewellery, including crosses may be worn - but underneath the uniform. Other airlines have the same policy.
"The policy recognises that it is not practical for some religious symbols - such as turbans and hijabs - to be worn underneath the uniform. This is purely a question of practicality. There is no discrimination between faiths.
"In Nadia Eweida's case, she is not suspended and we want her to come back to work. We have explained to her the need to comply with the uniform policy like all her colleagues whatever their faith."
BA said Ms Eweida had been offered a non-uniformed post were she would be able to openly wear her cross but had refused to take it.
She has seven days to lodge another appeal against the airline's decision.