"And they are girls who do not have the self-confidence and sense of individuality they would have if mums were saying no to them regularly and letting go much earlier on."
She added that BFF mothers negotiate with their children rather than setting out clear rules and consequences, adding: "Parents are delaying the assumption of authority that used to go along with being an adult."
The head stressed she had not noticed the trend at her school, but said she had seen it in previous teaching jobs and outside school. She said some mothers, who feared being tough on their children, had even asked her to telephone their daughters to wake them at 7am.
She told The Sunday Times: "One said 'I always make her a cup of tea in bed but she still doesn't get up.' I remember thinking 'How ridiculous.' That girl turned up late for her university interview. Quelle surprise."
And a lack of limits also appears to have an impact earlier in the girls' education. "Parents now look to us to set boundaries that are not available to their children at home," Mrs Brown said.
"Some rules are being laid down far too tentatively by parents. There is then a collision of practices between school and home."
She added that learning to be independent is a vital part of growing up. "That cannot happen easily if parents are all over their [children's] territory.
"When some universities provide beds for parents to stay during exam time and when daily Skype [calls] home and essays emailed for parental editing are routine, you realise that what we have grown here is Generation D - for "dependent". Dependent parenting... is a short cut to perfectionism and anxiety.' She also said that BFF mothers tend to be 'helicopter' parents, who take control of their daughters' academic and social lives.
"When a mother tells me, 'I am my daughter's best friend', I tend to smile with a lifted eyebrow. Probably that girl would have lots of friends her own age if her mother was not trying to be her best friend," Mrs Brown said.
The head - who campaigns for girls' education - lives with her playwright husband Ben and their two children. Earlier this year she called for teenage girls to be given a year off from exams so they had time to "make mistakes and fall in love".
- Daily Mail