A children's charity that pushes gender identity theory in schools has been accused of giving dangerous chest-flattening devices to young girls against their parents' wishes.

An investigation by The Telegraph alleges that Mermaids, a taxpayer-funded charity that says children can be born in the wrong body, offered to discretely send a breast binder to a girl it thought was 14.

This was after the girl told the charity her mother did not approve of the device. Chest-binding has been described as a form of self-harm and linked to breathing difficulties, chronic pain and other complications.

The Telegraph has also seen discussions on the charity’s moderated forum for 12 to 15-year-olds on how to raise money to start taking drugs and the best way to take testosterone.

Campaigners and MPs said on Sunday night that there were “massive safeguarding red flags” over Mermaids and called for an immediate investigation by the Charity Commission.

Stephanie Davies-Arai, the founder and director of Transgender Trend, which campaigns for evidence-based care for children, said: “Mermaids should not be sending breast-binders out behind parents’ backs.

"Parents are the primary caregivers and they have ultimate responsibility for their health and welfare. To exclude parents from really serious decisions like this is anti-safeguarding", she added.

"People have been raising concerns for years. The Charity Commission and anybody who is recommending Mermaids or using them for training needs to really seriously think about child safeguarding.”

Her call was echoed by MPs including Miriam Cates, who said that the charity “is stating an extreme position on gender ideology" and "going behind parents’ backs to speak to children".