Reforms to make relationship and sex education compulsory for pupils from the start of primary school are to be announced by the Department for Education.
But ministers were accused of holding the House of Commons in contempt after the announcement failed to appear at the expected time, despite widespread reports in the media. Officials would say only that the plans would be unveiled "in due course".
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 expected to be taught about the biological mechanics of sex.
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."
Labour's shadow education minister Mike Kane raised a point of order in the House of Commons about the Government's failure to inform MPs about its plans.
"The Department for Education briefed the media earlier today that they were planning to bring forward a change to the Children and Social Work Bill that would introduce statutory sex and relationship education for pupils from key stage one onwards," said Mr Kane.
"It was also my understanding that there would be a written ministerial statement outlining the update to that Bill.
"However, I now understand, once again from briefings to the press rather than to any statement oral or written to this House, that there will not be an announcement today.
"The House is being held in contempt here. This matter relates to a Bill that will return to the floor of the House next Tuesday.
"It has wide support across all sides. I think members need clarity from Government."