In the letter, Nick Fletcher, MP for Don Valley, said "boys are boys and girls are girls” and modern media glamorises a "transgender lifestyle" and affirms “something that is nothing more than a phase”.
The Tory MP added: “I understand schools sometimes feel as though they have to be all things to all people and are afraid of being classed as transphobic.
“But please be assured that a push back on this is desperately needed and if you do I will stand up for you at all times and champion your resolve.”
After sending the letter, he was criticised by several headteachers, LGBT+ organisations and local councillors, but said: “I stand by my views”.
Mr Fletcher added: “This was a private letter sent by me to the heads of all the schools in my constituency.
“In it I offered my support with regards to the increased numbers of young children who are expressing a view that they are a different gender to their sex.
“I have kept away from social media on this subject as I understand the negativity that can come when discussing this issue.
“I am therefore deeply disappointed that these private letters have entered the public forum. I am deeply regretful if this has caused any upset as that was not my intention.”
It comes after Culture Secretary Nadine Dorries urged other sports to follow the example of swimming’s world governing body FINA and bar transgender athletes who have gone through puberty from competing in women’s events.
FINA announced the decision after an extraordinary congress in Budapest earlier this week, adding that it will look to set up an open competition category in which athletes can compete irrespective of their sex or gender identity.
Dorries welcomed the move and indicated she is already in the process of bringing together leaders of other sports with a view to establishing similar parameters across the board.