New teaching reforms are set to make relationship and sex education compulsory for all secondary-age pupils, while primary schools will be required to teach children about relationships from the age of four.
An announcement on the changes is expected 'in due course' though it was thought unlikely to be unveiled by the Department for Education on Tuesday.
At present, sex education is compulsory only for secondary pupils in schools run by local authorities.
Reports suggest that the reform will make it mandatory in all schools, including academies, independent schools and religious free schools and extend the subject to include relationships as well as modern phenomena such as internet porn and sexting.
However, Education Secretary Justine Greening has previously said that any education will be 'age appropriate' and it is not expected that children as young as four would be taught about the biological mechanics of sex.
Instead, they are likely to be taught about relationship issues.
A Downing Street spokesman told reporters: "Relationship and sex education is clearly an important part of preparing children and young people for adult life.
"It is compulsory already in maintained secondary schools and the Education Secretary has made clear this is an area she wants to look at.
"You can expect something from the department in due course."
The move was welcome by Rotherham MP Sarah Champion.
She said: "If we don't have the confidence to give our children proper education about healthy relationships, about boundaries then they are going to go online and find it.
"What they find when they type in sex is porn. Do we really want our children to be educated by porn?"
She said if we do not educate children in schools then they will go online to access information about sex, and more than likely come across pornography.
Tanya Barron, chief executive of children's charity Plan International UK, said: "Sex and relationships education is about ensuring that children are equipped to deal with the reality of 21st century life.
"We know, for example, that with the internet and smartphones, children can access potential harmful material and be pressured into sharing sexualised messages.
"Good quality, age-appropriate sex and relationships education is a common sense policy to help children deal with those situations. It's what children themselves have been asking for, and it's what parents want too.
"More than three-quarters of parents we surveyed said they felt children should be educated about the impact of pornography, while 73 per cent wanted them to learn about the dangers of sexting."