"I think it's appalling and it should be a matter of anxiety and concern to everyone who cares about the future of this country," he told The Observer.
The Prime Minister appeared to acknowledge that recent initiatives aimed at discouraging teenage parenthood - including hostels for teenage mothers, helplines and new education and training opportunities, had little relevance to younger school-aged children.
Sex education in schools was partly the answer, he said, though parents should also take responsibility for their 12-year-old children by keeping them off the streets at night.
"We need to find a new national moral purpose for this new generation. People want to live in a society that is without prejudice, but is with rules, with a sense of order. Government can play its part, but parents have to play their part. There's got to be, if you like, a partnership between Government and the country to lay the foundations of that moral purpose."
While Tory leader William Hague accused Mr Blair of pursuing policies which were neither moral nor a crusade, and others warned of the danger of Labour becoming embroiled in its own "back to basics" fiasco, the prime minister refused to back down.
At the same time, however, ministers were insistent that his remarks were not a call for a return to Victorian values or an attempt to tell adults how to run their lives.
Downing Street sought to build on The Observer platform by announcing that boys as young as 13 would be pursued by the Child Support Agency to provide financial support if they fathered babies. And once again it indicated that schools would be asked to play a central role, with new curriculum guidelines recommending that teachers instruct pupils from the age of seven upwards about the value of marriage, traditional two-parent families and the responsibilities of bringing up children.
The guidance is included in proposals for a revised national curriculum, due to take effect in schools in a year's time. David Blunkett, Education and Employment Secretary, has made a number of last minute changes to satisfy traditionalists.
In English, secondary teachers will have to select two pre-First World War works of fiction from a prescribed list of 19 authors, and four works of pre-1914 poetry from a list of 28 poets.
But, despite some press reports, figures such as Henry VIII and Elizabeth I, and events such as the Battle of Hastings will no longer be part of the statutory curriculum, though they are recommended for study.
Battle-lines were further drawn this week on one of the issues which most clearly divides Labour and the Tories, with William Hague, who is himself a comprehensive schoolboy, saying that grammar schools should be set up in every town if there is a demand from parents. While promising a big expansion of specialist schools, David Blunkett has pledged that there would be no new selective schools under Labour.
Last week, anti-grammar school campaigners announced plans to take advantage of parental ballots to try to force existing grammars to abandon selection.