The introduction of mandatory sex education in Britain, including teaching about same-sex relationships and transgender identity, has sparked a backlash from religious parents and faith schools that want to be able to opt out from the new rules.

Sex education will become compulsory from the age of 12 in September 2020, according to new government guidelines published late on Monday. Previously parents and faith schools had been able to opt out.

Under the new guidelines parents can still take their children out of sex education lessons until the year before their 16th birthday.

Children will also learn about same-sex relationships before they turn 12 and that stereotypes based on gender and sexuality can be damaging.

More than 111,000 people signed a petition arguing for the "fundamental right" to opt out of relationship and sex education, triggering a debate in Britain's parliament on Monday.

"We have grave concerns about the physical, psychological and spiritual implications of teaching children about certain sexual and relational concepts... and believe that they have no place within a mandatory school curriculum," the petition said.

"Parents need to have a right of withdrawal from sex education and this is one of the reasons why home education is growing," said Clive Ireson, the head of the Association of Christian Teachers, a lobby group. Read more via Reuters