The campaign, which aims to reach 40 million people, shows a scan of a foetus with a halo above its head. It will feature on billboards over Christmas and reads: "He's on his way: Christmas starts with Christ."
Drawn up by ecumenical charity, Churchads.net, the "Ultrasound Jesus" campaign is backed by a number of Christian organisations including the Church of England, the Baptist Union, the United Reformed Church, the Anglican and the Methodist churches.
The National Secular Society (NSS) has criticised the ad, saying it gives the impression that it is politically motivated.
"It is an incredible piece of naivety on their part," said Terry Sanderson, director of the NSS. "If they are hoping to stop the secular drift away from Christmas as a Christian festival, they risk doing the opposite. It gives the impression that it was politically motivated, that they are trying to put across some sort of subliminal message. The image is too specifically associated with pro-lifers to be seen in a benign context. They should go back to angels and cribs."
He said: "The advert is saying that Jesus was alive as a person before he was born. They have a halo round his head and you don't have a halo around the head of a blob of jelly or a cluster of cells. This is not a cluster of cells but a human person and it just happens to be the God man Jesus. It is about the humanity of the unborn. That is a very, very powerful statement that will strike a chord with the general population."
Mike Elms, vice-chair of ChurchAds.net, denied the ad was in any way political and said that its sole purpose was to convey the Christian message of Christmas in a modern, secular context.
"We wanted to convey that Christmas starts with Christ. That this baby was on the way. Then we thought that the scan was a way of conveying that: it is modern currency in announcing a modern birth. We put a halo on it because theologians speak of Jesus being fully human and fully divine."
Elms added: "People are entitled to talk about it, but when the posters are put up, from the 6 till 20 December, it will be seen in context and its real message will become clear."
ChurchAds.Net, previously known as the Churches Advertising Network (CAN), has a council of reference which includes two Church of England bishops and leaders from across the church denominations. The Roman Catholic church is not part of this body.
Ultrasound Jesus was created with the help of Chas Bayfield best known for his award-winning blackcurrant Tango advert. It follows a poster campaign last year, where paintings by artist Andrew Gadd depicted the nativity scene in a bus shelter.SIC: TGUK