More than 24,000 UK schools take part in Operation Christmas Child, in which pupils fill shoeboxes with toys, clothes and toiletries. Organiser Samaritan's Purse last year collected more than a million shoe-boxes in the UK which were delivered to 18 countries including Afghanistan, Albania, Bosnia and the Ukraine.
Describing itself as an "evangelical Christian organisation", its website says: "This programme provides an opportunity to be involved in a simple, hands-on project while focusing on the true meaning of Christmas - Jesus Christ, God's greatest gift."
Marilyn Mason, education officer for the British Humanist Association, said: "It is important for children to have opportunities to help others. But I am certain that many parents and teachers would prefer to make such gifts with no religious strings attached."
Samaritan's Purse said its main purpose was not conversion and that it did state it was a Christian organisation on promotional literature for schools. UK director David Applin, an Anglican clergyman, said: "We take many boxes to Muslim countries where we would be arrested if they contained Christian messages."
He added that he understood why some phrases on the charity's website "might provoke a reaction".