New guidance on political impartiality in the classroom is confusing and likely to scare teachers in England away from tackling important subjects such as climate change and racism, according to education unions.

The guidance issued by the Department for Education says that recent historical events “which are particularly contentious and disputed”, such as “many topics relating to empire and imperialism”, should be taught “in a balanced manner”.

The DfE’s guidance singles out the Black Lives Matter movement, saying that while teachers should be clear “that racism has no place in our society”, the demands of campaigning organisations such as BLM “go beyond the basic shared principle that racism is unacceptable”.

In teaching of scientific facts around climate change, teachers are told they should not provide balance in the form of misinformation or unsubstantiated claims.

However, the guidance goes on to state that “where teaching covers the potential solutions for tackling climate change, this may constitute a political issue”.

Nadhim Zahawi, the education secretary, said: “I don’t want there to be any barriers – real or perceived – to teachers’ vital work in this space, which is why I am reinforcing that no subject is off-limits in the classroom, as long as it is treated in an age-appropriate way, with sensitivity and respect, and without promoting contested theories as fact.”

But Mary Bousted, joint general secretary of the National Education Union, said that the guidance “does not so much clarify existing guidance as add new layers of mystification and complexity to it” for teachers and school leaders.

“This could induce such a level of uncertainty and caution in schools about ‘political issues’ that they are less likely to engage with them,” Bousted said.

“The losers in the DfE’s 34-page game of obfuscation about what is and is not a ‘political’ issue will be the students who are denied the opportunity to engage with the most challenging issues of our time.

“The warning lights that the government is flashing around climate change, racism, world poverty and the legacy of empire as topics of exploration are more likely to decrease students’ engagement with learning than to stimulate it.”

The guidance – which contains no new statutory requirements and is based on existing legal duties – avoids defining “political issues”, stating that ethical debates are not political issues if they are “shared principles that underpin our society”, such as freedom of speech or challenging racism.

Instead, school leaders and teachers are told to use “reasonable judgment to determine what is and is not a ‘political issue’”.

The guidance also warns against reliance on teaching material provided by external agencies. In one scenario of teaching about the Israeli-Palestinian conflict, the document says teachers need to ensure that any external resources used are impartial.

When teaching younger pupils about “significant” political figures, “including those who have controversial and contested legacies”, schools are advised “to focus on teaching about what these figures are most renowned for and factual information about them if teachers think pupils may not be able to understand the contested nature of more complex analyses of their lives, beliefs and actions”.

More complex discussions about controversial figures “might be reserved for older pupils who are more likely to be able to understand and engage in this debate”.

The Association of School and College Leaders said it would be “studying the guidance carefully to understand its implications”.

“The vast majority of teachers are very good at managing these discussions in a way that is balanced and impartial. We welcome anything which helps them to navigate this difficult territory. However, we are keen that this should not be over-prescriptive as it could have the unintended consequence of deterring open discussions,” the ASCL said.