The Advertising Standards Authority (ASA) announced today it had banned the ad for the Antonio Federici brand for "making a mockery" of the beliefs of Roman Catholics.
The advert, which appeared in The Lady and Grazia magazines earlier this year, showed the heavily pregnant nun standing in a church holding a tub of ice cream and a spoon, with text stating "Immaculately conceived" and "Ice cream is our religion".
Ten readers complained that the advert was offensive to Christians, particularly those who practised Catholicism.
A spokeswoman for Antonio Federici said the company intended to defy the ban by publishing another advert portraying "a continuation of the theme".
She said: "We intend to defy the ASA's ban and will publish another advert from the series before the Pope's visit later this week.
"We are in the process of securing a series of billboards close to and along the planned route of the Pope's cavalcade around Westminster Cathedral."
The ASA said its rulings "must be followed".
A spokesman said: "ASA rulings must be followed and we are taking steps to ensure Antonio Federici do so.
"We do not comment on the likely compliance of ads that have not yet appeared. However, we are continuing to conduct work behind the scenes, including with the advertiser, to ensure they comply with the rules."
Defending the banned ad, Antonio Federici said the idea of "conception" represented the development of their ice cream, and their use of religious imagery stemmed from their strong feelings towards their product.
The company said it also wished to "comment on and question, using satire and gentle humour, the relevance and hypocrisy of religion and the attitudes of the church to social issues".
The publishers of The Lady received eight complaints made directly to them and said that, in hindsight, it had been a "misjudgement" on their part to publish the ad.
The magazine told the ASA that it "regretted the offence that had been caused to readers", adding that it would not publish the ad or anything similar to it in future.
Grazia magazine said it considered that the advert was intended to be lighthearted and not mocking of any religious groups.
The ASA noted that advertising rules stated ads "should contain nothing that is likely to cause serious or widespread offence".
It said: "We considered the use of a nun pregnant through immaculate conception was likely to be seen as a distortion and mockery of the beliefs of Roman Catholics.
"We concluded that to use such an image in a lighthearted way to advertise ice cream was likely to cause serious offence to readers, particularly those who practised the Roman Catholic faith."
The number of complaints was relatively small, but the advert had been placed in only a small number of publications.
It ruled that the advert must not appear again in its current form.
The ASA banned another advert for Antonio Federici in July last year that showed a priest and a nun looking as though they were about to kiss.
SIC: BT/UK