THE LEBANESE Muslim Association claims it organised for a festive message be written in the sky above Lakemba Mosque following reports of a fatwa being declared on Christmas.

The message, written in white in the sky above the nation's biggest mosque reads : Merry Xmas, the ABC reports.

The sky message comes after an Imam at Australia's biggest mosque reportedly issued a fatwa against Christmas, warning followers it is a ''sin'' to even wish people a Merry Christmas.

The ruling, which followed a similar lecture during Friday prayers at Lakemba Mosque, was posted on its Facebook site on Saturday, according to media reports.

The mosque's Facebook Page was hit with more than 100 posts, including numerous sarcastic "Merry Christmas and Happy New Year".

One Facebook user, Doug Barclay, wrote: "How do I issue a fatwa against the Easter Bunny?", while another, Brett Rann, said: "Don't pretend Islam is tolerant of other religions".

It appears the post is no longer on the page and the Islamic Friendship Association says the fatwa isn't valid.

"These types of comments, unfortunately, do nothing to promote unity, and I'm glad they've been taken down," the association's spokesman Keysar Trad told the ABC.

"When I saw it I was quite shocked ... It just gives people the wrong impression," Mr Trad said.

"I'm happy that they are not sticking by those comments and that they are distancing themselves from them," he added.

The original post came from text copied and pasted from the internet by junior members of staff and the mosque did not endorse the message, the Lebanese Muslim Association said.

Samier Dandan, the president of the association told ABC television the Facebook post was a mistake by a youth officer.

"We have given him a warning," he said.

The head imam at Lakemba, Sheikh Yahya Safi, reportedly told the congregation during Friday prayers they should not have anything to do with Christmas.

The fatwa reportedly warned: ''Disbelievers are trying to draw Muslims away from the straight path.''

It said Christmas Day and associated celebrations are among the ''falsehoods'' for a Muslim to avoid.

''Therefore a Muslim is neither allowed to celebrate the Christmas Day nor is he allowed to congratulate them,'' it said.

The fatwa has been condemned by other Muslim leaders. The Grand Mufti of Australia, Ibrahim Abu Mohammad, was quoted by Fairfax media as saying the foundations of Islam were peace, co-operation, respect and holding others in esteem.

"Anyone who says otherwise is speaking irresponsibly," it quoted him as saying.