The girl was at primary school when she conceived last summer and had a daughter last weekend, the Sun reported.
The girl cannot be identified for legal reasons and lives with her 27-year-old mother, one of the UK’s youngest grandmothers, in North London.
The couple plan to bring up their daughter together, a source said.
The girl, a pupil in year seven, had the baby last Sunday evening with her boyfriend, who is just 13 and a year nine student at another school.
The pair have the lowest combined age of any British parents in history and at 12 years three months, the girl is five months younger than the nation’s previous youngest mother.
The new parents have been in a relationship for more than a year. Both sets of grandparents are said to be “incredibly supportive”.
A source told the Sun: “It’s a very difficult situation because the parents are so young. But their families are right behind them.”