Lessons incorporating same-sex relationships and transgender issues have resulted in protests outside primary schools, children being removed from classes and head teachers being threatened. BBC News looks back at the origins of the Birmingham LGBT teaching row.
How did it all begin?
The No Outsiders project was the brainchild of Andrew Moffat, assistant head teacher at Parkfield Community School in Birmingham.
Mr Moffat created it to teach children about the Equality Act 2010 and British values. He also wanted pupils to "be proud of who they are while recognising and celebrating difference and diversity".
The project used books about a dog that doesn't feel like it fits in, two male penguins that raise a chick together and a boy who likes to dress up like a mermaid.
"It's about teaching young children that we are different in reception and year one, that's as far as this work goes," he said. "We're just talking about being different and being friends."
In 2014, the project was piloted at his school in the Alum Rock area of the city and was soon adopted by schools across the country.
Three years later, Mr Moffat was made an MBE for services to equality and diversity in education.
When did controversy begin to unfold?
In January this year a parent whose child attends Parkfield school raised a petition, claiming the teaching contradicted the Islamic faith.
"Children at this age don't even know if they are coming or going, let alone knowing what sexual orientation they will become," Mariam Ahmed said.
While the writing in the Koran is open to interpretation, it includes passages which suggest homosexuality is against the will of Allah.
Meetings took place between Mr Moffat and concerned parents, but some became "personal and aggressive", the school said in a statement.
Within days, some parents began protesting outside the school at home time. Several pupils were also kept at home.
How did the school respond to the growing anger?
The No Outsiders lessons were paused to allow teachers to "re-engage with our parents", Mr Moffat said.
Four other schools in the city - Leigh Primary School, Alston Primary School, Marlborough Junior and Infants School and Wyndcliff Primary School - halted their lessons.
The protests, however, did not subside. They began to be held daily.
Mr Moffat, who is gay, told the BBC he had been accused of promoting "personal beliefs" and had received "nasty emails" and threats, including one which warned he "wouldn't last long".
Protests spread to other schools in Birmingham, including Anderton Park Primary School, in Balsall Heath.
Its head teacher Sarah Hewitt-Clarkson told the National Association of Head Teachers' conference protesters had waved banners with slogans such as "Adam and Eve, not Adam and Steve" and "We have a say in what they learn".
"The lead protesters have no children at my school," she disclosed.
Who is heading the demonstrations?
Amir Ahmed has co-ordinated protests outside seven Birmingham primary schools and used a loudspeaker to address parents and other campaigners at Parkfield.
Although he does not have children at any of the affected schools, he said he was motivated by his religious beliefs and concerns about what children in his community were being taught.
"We are a traditional community - we have traditional family values and morally we do not accept homosexuality as a valid sexual relationship to have," he told the BBC.
"We do not believe in homosexuality but that does not make us homophobic."
He told BBC Newsnight the lessons were "not about gay lesbian rights and equality". Mr Ahmed added: "This is purely about proselytizing homosexual way of life to children."
When asked if children could be recruited to become gay, he said: "Well you can condition them to accept. This has been a normal way of life. And it makes the children more promiscuous as they grow older."
The protests outside Anderton Park have been led by property developer Shakeel Afsar. He got involved after his sister's son brought home a book about a boy who wants to dress up as a girl.
He said critics had "put parents into the bracket as if we are homophobic - that's far from the truth".
"All we are concerned is we are having our children come home with material that contradicts our moral values that we are trying to instil in our children."
Where have the protests spread?
An investigation by BBC Newsnight found letters opposing the teaching of relationships and sex education (RSE) and LGBT equality had been sent to schools across the country.
Schools in Birmingham, Bradford, Bristol, Croydon, Ealing, Manchester, Northampton and Nottingham had received the letters from conservative Muslims.
In Kent, schools received letters from Christian parents and there have been reports that some children in Bristol have been removed from school by concerned parents.
What do education chiefs say?
Ofsted has backed the No Outsiders programme, with its chief inspector Amanda Spielman saying all children must learn about same-sex couples regardless of their religious background.
She said it was crucial children were exposed to differences in society, and important children knew "there are families that have two mummies or two daddies".
An Ofsted report also found there was no evidence the curriculum at Parkfield overly focused on LGBT issues or was not taught in an age-appropriate manner.
Meanwhile, Education Secretary Damian Hinds said primary schools should be able to choose what they teach about same-sex relationships, if they "consider it age appropriate".
In a letter to head teachers he suggested listening to and understanding the views of parents as a way schools can "increase confidence in the curriculum" to help children leave school "prepared for life in modern, diverse Britain".
He said head teachers should consult parents but he reassured them parents had no right to veto what was taught.
The Department for Education says lessons about relationships will become compulsory from September 2020.
How do teachers feel?
The prolonged action has taken its toll on several teaching staff.
Hazel Pulley, chief executive officer of the trust which runs Parkfield school, said some staff had lost weight and could not sleep.
An emotionally-charged meeting in April between 85 head teachers, Department for Education (DfE) officials and the council in Birmingham saw some people in tears.
"We feel completely alone here and feel as if we're getting no overt support whatsoever from the government," one head, who wanted to remain anonymous, said.
Rob Kelsall, the national secretary of the National Association of Head Teachers, tweeted that the "government need to step up and sort this out".
Like Mr Moffat, the head teacher of Anderton Park says she has been repeatedly threatened in emails and phone calls.
"There's a whole variety of emotions: embarrassment for lots of our community and our parents who think this is just awful what's happening; frustration that it's going on so long; frustration that great British laws like 'you can protest peacefully' actually are causing us a problem," Sarah Hewitt-Clarkson said.