Carol Monaghan, a Catholic and the SNP MP for Glasgow North West, was speaking in a Commons select committee when she was pictured on parliamentary television with the Christian symbol.
Some believers choose to have a cross marked with ashes on their foreheads on Ash Wednesday to symbolise the beginning of Lent, publicly display their faith, and remember that ultimately they will die and 'return to the ashes' they came from - a reminder of mortality and the need for God.
Bishop of Willesden, Rt Rev Pete Broadbent, thought little of the question BBC Politics asked on Facebook:
He responded later:
Archbishop Cranmer, a Christian blogger, argued in an article that the BBC's question made little sense given that it has never previously questioned when other Christians, Muslims or Sikhs have worn or been marked with religious symbolism in Parliament.
He said: "Was it appropriate for the BBC to make such a public display of its Christian cultural ignorance, its shocking religious illiteracy, and its crass insensitivity toward all Christians in public service?"
Carol Monaghan herself has said she's not ashamed to publicly be known as a Christian and that she wants to get on with her work as a politician. As discussions about her cross began, she tweeted:
Surprised by fuss over my ashes. Always done this but I guess more interest in me as an MP. All comments positive https://t.co/fYdU2wwX3s— Carol Monaghan MP (@CMonaghanMP) March 2, 2017
People spoke out in support of her on Twitter:
Good for Carol Monaghan MP. There is no shame in hiding who you are or what you believe in. https://t.co/QwRbPffBov— Claudia Mendoza (@Claud_Mendoza) March 5, 2017
MP Carol Monaghan; I salute your example and leadership to those who fear displaying their beliefs because a non-christian may be offended!— Bob Graham (@bobgraham54) March 5, 2017
The BBC said: "The Facebook post was meant to attract the audience attention and to encourage them to read the article."