The warning came as Ms Mordaunt's Department for International Development announced an additional £21 million package of support for a United Nations fund enabling agencies to respond to emergencies around the world.
The department cited ongoing famines and conflicts in Yemen, South Sudan and Burma as significant concerns.
In early 2017 the United Nations said the world faced the largest humanitarian crisis since 1945, with more than 20 million people in four countries facing starvation and famine. But Ms Mordaunt believes next year "could be even worse".
“While 2017 was a year of harrowing humanitarian crises, the truth is 2018 could be even bleaker," she said.
“When we see suffering, we instinctively want to help. Britons are big-hearted, open-minded and far-sighted – qualities that define a great nation.
“This year, through UK aid and further public donations, we helped avert famines in Nigeria and Somalia, gave emergency help to the survivors of the Caribbean hurricanes and provided a vital life-line to people suffering from conflict in Syria and Yemen.
“Britain is giving life saving aid, but also hope, to millions of people around the world. In the challenges 2018 brings Britain will continue to be at the forefront of the global humanitarian response."
The £21 million boost to the UN's Central Emergency Response Fund will help provide clean water and sanitation to 13 million people and food to nine million people, officials said.
It comes in addition to the UK's annual £55 million contribution to the Central Emergency Respond Fund, which officials said saved lives in Syria last year as well as supporting people suffering from violence at the hands of Boko Haram, the militant jihadist group based in Nigeria.
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