Some schools avoid teaching the Holocaust and other controversial history subjects as they do not want to cause offence, research has claimed.
The Holocaust will be protected in the new curriculum
Teachers fear meeting anti-Semitic sentiment, particularly from Muslim pupils, the government-funded study by the Historical Association said.
It also said the way the slave trade was taught could leave both white and black children feeling alienated.
Ministers in England had asked for guidance on teaching emotive subjects.
When he commissioned the report last year, schools minister Lord Adonis said the national curriculum encouraged teachers to choose content "likely to resonate in their multicultural classrooms" - but some found it difficult to do that.
The Historical Association report claimed: "Teachers and schools avoid emotive and controversial history for a variety of reasons, some of which are well-intentioned.
"Staff may wish to avoid causing offence or appearing insensitive to individuals or groups in their classes.
"In particular settings, teachers of history are unwilling to challenge highly contentious or charged versions of history in which pupils are steeped at home, in their community or in a place of worship."
The report gave the example of a history department in a northern city which decided not to teach the Holocaust as a topic for GCSE coursework.
It cited another school which taught the Holocaust, but then avoided teaching the Crusades because "balanced treatment" of the topic would have challenged what some local mosques were teaching.
Emotive issues such as the slave trade can be taught too blandly, portraying Afro-Caribbeans as victims and isolating black children, the report said.
But when teachers downplay the role of the white authorities in abolishing the slave trade, white children can become alienated.
The report called for resources, which were scarce at present, to be made available to teach controversial and emotional history subjects.
Initial teacher training should include more attention on how to teach these subjects and a better research base should be made available to teachers, it said.
And further research into the issue, particularly the attitudes of different groups, families and individuals' to difficult subjects, needed to be carried out.
A government review of citizenship education recommended that all pupils should learn about issues such as slavery and the legacy of the British Empire.
A Department of Education and Skills spokesman said there was scope for schools to make their own decision on what to teach within the national curriculum
But he added: "Teaching of the Holocaust is already compulsory in schools at Key Stage 3 [age 11-14].
"It will remain so in the new Key Stage 3 curriculum from September 2008.
"As Alan Johnson made clear in January there are certain subjects which will be protected in the new curriculum and that includes the Holocaust."
The department and Understanding Slavery have launched a citizenship resource and a national competition, debate and showcase for Key Stage 3 students to explore the legacies of the transatlantic slave trade.
A Commission for Racial Equality spokesman said the Historical Association report painted a "worrying picture".
"The teaching of history provides the perfect forum for stimulating the development of shared values that are essential if everyone is to contribute and play a full part in an integrated British society."
It was essential that teachers were supported in developing the confidence and expertise to discuss all historical periods and events in a balanced and sensitive way, the spokesman said.