Schools can delay teaching the new relationships and sex education (RSE) curriculum until the start of the summer term in 2021, the Department of Education has said.
From September, relationships education is set to become compulsory in England's state primary schools, and relationships and sex education will be compulsory in secondary schools.
The decision to allow schools to delay starting RSE has been taken to give them more time to prepare to deliver the new curriculum and to engage with parents on their policies and content following lengthy school closures amid the Covid-19 crisis.
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Teaching health education, which will also become compulsory for all state schools in England from September this year, can also be postponed.
Schools that have not yet been able to meet requirements in statutory guidance because of "lost time and competing priorities" can delay teaching these compulsory subjects for the first two terms of the school year while they make preparations, according to a letter from the DfE.
But schools that already feel ready to deliver the new curriculum are being encouraged to start lessons from 1 September, or ideally within the first few weeks of the academic year.
A letter from the DfE suggests that schools take a "phased approach" if needed when introducing the subjects to ensure teaching starts as soon as possible.
Schools should consider prioritising lessons on mental health and wellbeing, as this curriculum content will be "important as pupils return to schools" following lengthy periods at home, it says.
The decision comes after a poll, published by Bett in January, suggested that more than half of teachers did not feel prepared to teach sex and relationships education to children.
Last year, some primary schools in Birmingham faced protests at the school gates from parents who opposed teaching children about the existence of LGBT+ relationships.
Mary Bousted, joint general secretary of the NEU teaching union, said: "It is unfortunate that these reforms have had to be postponed in light of the wider challenges facing our schools.
"However, we hope that this additional time will support schools to develop their curriculum in line with the new guidance and ensure they have access to the training, guidance and resources promised by the DfE before the outbreak."
James Bowen, director of policy at school leaders' union NAHT, said: "The new relationships, sex and health education curriculum has been a long time coming, and we would like to see it implemented as soon as possible for the benefit of all pupils.
"However, the Covid-19 outbreak has meant that many schools have not been able to do all of the planning, staff training and engagement with families necessary ahead of getting teaching under way in September.
"These are all vital elements of delivering the new curriculum successfully, so we are pleased that schools are being given additional time to prepare properly."
Julie McCulloch, director of policy at the Association of School and College Leaders, said: "This flexibility is a prudent move in the circumstances, given the lost preparation time as a result of the shutdown of schools since March.
"It will help ensure schools are able to put in place the materials and training which are necessary to deliver this important learning effectively."