Hindus are demanding that Royal Mail withdraws one of this year's Christmas stamps, claiming the mother and child image it represents is insulting to their religion.
The 68p Christmas stamp, which would be used to send mail to India, features a man and woman with Hindu markings worshipping the infant Christ.
The image is one of a series of six mother and child stamps that went on sale this week and would be used on cards sent to India.
Ramesh Kallidai, secretary general of the Hindu Forum of Britain, said the image was insensitive, because it showed people who were clearly Hindu worshipping Christ.
He has now asked its members, and members of other Hindu groups, to send unstamped protest letters to Royal Mail's headquarters.
He said he hoped this would cause a "logistical nightmare worse than withdrawing the stamp". The group, Britain's largest Hindu body, is also planning a mass protest outside the headquarters.
"It is the equivalent of having a vicar in a dog collar bowing down to Lord Ram on a Diwali stamp," he said. "These things need to be done with sensitivity."
The stamps were drawn from religious images around the world. The Hindu stamp was taken from a picture that hangs in Bombay, India, and was painted in the 17th century.
It is an Indianised version of a European print of The Holy family with St Anne and the two angels, according to the city art gallery. It has a European theme but a Mughal setting.
The entire picture shows St Joseph trying to push aside a huge curtain so that St Anne can behold the baby.
The picture was chosen for Royal Mail by this year's stamp designer, Irene Von Treskow, an Anglican priest in an English-speaking church in Berlin.
She said she was fascinated by the image because it was so interesting to see a Mughal painting with a Christian subject.
She does not believe the picture is offensive. "How can it be?" she asked. "It is 17th-century art."
She said she found the painting in a book and then looked up the image on the internet.
Mr Kallidai said the man in the painting has a "tilak" marking on his forehead, clearly identifying him as a Vaishnava Hindu.
The woman has the traditional "kumkum" mark on her forehead, identifying her as a married Hindu woman. "These are exclusively used by Hindus," he said.
"While many people doubt the authenticity of the age of the painting, we believe that even if this were true, it would be insensitive to use it at a time when the issue of conversions in India has been a subject of heated debate.
"Even if we accept that an artist in 1620 made the mistake of portraying practising Hindus worshipping the infant Christ, we should be asking if this is politically and communally correct in the 21st century."
He said he had called for Royal Mail to issue a Diwali stamp for the Hindu festival of light, which began on Monday, but there had been no response.
"It is striking to see that Royal Mail thinks it prudent to issue Christmas stamps that can cause resentment in the worldwide Hindu community but remains silent on the issuing of stamps for Diwali, the festival of lights celebrated by the third largest faith community in the UK and by a billion Hindus worldwide."
Royal Mail said no offence had been intended. "We thought it would be nice to return to a religious theme," a spokesman said.
It was however sorry for any "inadvertent upset" caused, but there are no plans to withdraw the stamp.
"Many have already been sold. Royal Mail is, of course, more than willing to meet representatives of the Hindu community and we will apologise personally to them for any unintentional offence caused."
However, Hindu leaders said this response was unacceptable. "We cannot accept the Royal Mail argument that the stamps cannot be recalled," said Ratilal Chohan, the general secretary of the Hindu Council of the North.