Washington, US - For some, it is the heartwarming tale of two male penguins raising a chick together, but children's book And Tango Makes Three is also one of the most controversial texts in the United States, librarians say.

The illustrated book, which is intended to teach young children about gay parents, tops the 2009 list of 'most challenged titles' that the American Library Association (ALA) compiles as part of its annual Banned Books Week.

Individuals and groups in at least 15 US states have challenged libraries over the book, seeking to have it labelled with a content warning, moved to a different section of the library or removed from shelves altogether, according to the ALA's Office of Intellectual Freedom.

For Ms Deborah Caldwell Stone, the office's acting director, challenges to And Tango Makes Three and other books illustrate the importance of Banned Book Week, which will be celebrated from Sept 26 to Oct 3.

The event was first organised in 1982 to highlight the fact 'that challenges and banning are still taking place in this country on a regular basis, that books are removed from libraries because a person disagrees with the content,' she said.

'We estimate that we hear only about 25 per cent of the challenges. A parent comes in, complains, the book is removed from the library and we never hear about it and nobody reports it to us.' Of those challenges that are reported, she said objections are increasingly 'to either content that deals with gay themes or sex'.

On this year's ALA list of books is the Gossip Girl series, which has been described as 'Sex And The City for the younger set', and The Perks Of Being A Wallflower, which a parents group has listed as containing references to suicide, illegal drug use, teen sex and bestiality.

Mr Lewis Lipsitt, the founder of Brown University's Child Study Center, acknowledges that some literary content can scare or upset children.

But he warns that parents should not excessively restrict the access of their children to challenging material.

'There's no good evidence that the things children read are going to have a deleterious effect on them. Rather, they learn about behaviour of great diversity.'