She became pregnant at the age of 11 and gave birth to a girl weighing 7lb 4oz on Sunday.
Being 12 years and three months old makes her five months younger than the previous youngest mother, Tressa Middleton, who gave birth in Edinburgh in 2006.
The new mother was 10 when she met her boyfriend.
The pair, from north London, cannot be named for legal reasons.
The schoolgirl lives with her mother, who is 27, and is pledging her support to the couple.
A source is reported to have said, "The baby's mum and dad have been in a relationship for more than a year, so this isn't a fleeting romance. They intend to stick together and bring their daughter up together. They're very into each other, totally in love. She's in Year 7, he's in Year 9 at a different school."
Hilary Pannack, chief executive of teenage pregnancy charity Straight Talking, said: "I know girls who've been pregnant at 13 and have had the baby, and I've heard of 12-year-olds before.
"It costs £100,000 to the taxpayer to support the average teenage mother in the first five years - it is a reason but is not the primary reason to stop teenage pregnancy, which is that we are talking about young people's lives. We need to stop the cycle of teenage parents having children who are more likely to become teenage parents themselves."