The regulator's inquiry comes after previous engagement with the charity in 2015/16 when there was an anonymous complaint about the charity’s response to claims against senior staff and a serious incident report from the charity about misconduct and harassment claims.
The charity has been hit by a number of revelations in recent months about past sexual misconduct by former senior employees, including inappropriate texts sent to sent to female staff members by the charity's former chief executive Justin Forsyth.
The allegations about Forsyth were investigated by the charity in 2011 and 2015, and Forsyth left his role in 2015.
There have also been allegations against Brendan Cox, the charity’s former policy director, of inappropriate behaviour towards women.
Cox, the widower of the murdered MP Jo Cox, left Save the Children in 2015 and admitted last month that he had "made mistakes" during his time at the charity and apologised "deeply and unreservedly" for his past behaviour.
The Charity Commission said in a statement today that it received "direct assurance" from Save the Children’s trustees in 2015/16 that the recommendations of an independent review into the claims reported to the regulator had been acted upon.
But the regulator said further information had come to light in recent weeks about the 2015 claims and, after further engagement with the charity, the commission has decided to launch a statutory inquiry.
The commission said the inquiry would focus on whether Save the Children adequately reported the full extent of the 2015 allegations to the commission and the charity’s decision-making since February 2018.
How the charity handled various complaints made between 2012 and 2015 and reviews by the trustees of the charity’s response to the allegations will also be examined as part of the inquiry.
Michelle Russell, director of investigations and enforcement at the Charity Commission, said the commission had "questions that must be answered" and added that "we need to hold the charity formally accountable for providing them in a clear and timely manner".
In a statement, Save the Children said it would cooperate fully with the Charity Commission and "act swiftly on any lessons that emerge".
Peter Bennett-Jones, chair of Save the Children UK, said: "It is critical that Save the Children works with the Charity Commission to examine whether mistakes were made in the past. If mistakes were made, they will be fully acknowledged and properly addressed.
"We are committed to working with the Charity Commission to establish a truthful and accurate account of events and the charity’s response."
Bennett-Jones added that the charity was "unequivocally committed to building a workplace culture based on mutual respect – a culture in which all our staff feel protected, supported and listened to".
Save the Children established an independent review led by Suzanne Shale, an expert in organisational ethics, in February 2018 to strengthen the charity’s workplace culture.
The charity said in a statement that its new leadership team had strengthened staff protection systems through mandatory training and other measures, and that the charity had "policy of zero tolerance" towards bullying or harassment.