At a meeting in Rome's Vatican city with senior aides to Pope Francis, Ms Mordaunt highlighted the need for young girls to have access to contraception to avoid "the tragedy of 800 girls and women unnecessarily losing their lives every day through pregnancy or childbirth complications", the Telegraph reported.
She added: "Everyone deserves the right to a safe childhood, to an education and to a life without fear. For many girls this is not the case.
"Child marriage and a lack of control over their own bodies or access to reproductive healthcare including contraception, means many girls have no hope of completing education.
"It is crucial we engage with faith leaders to help us challenge deeply held beliefs and attitudes. The Catholic Church can help us in that and my appeal to them was to help us save lives, especially of young mothers."
Ms Mordaunt also raised the issue of LGBT rights at the meeting on Thursday with the Holy See - the global governing body of the Catholic Church.
The Catholic Church's strict rules regarding contraception were relaxed when an interview in 2010 with Pope Benedict was published in the book Light of the World: The Pope, the Church and the Signs of the Times.
In it he said condoms were acceptable to use when the sole reason was to "reduce the risk of infection" from Aids.
A spokesperson for the Catholic Church in the UK said Ms Mordaunt's comments were a matter for the Vatican, which is yet to publicly respond to the plea.
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