An appeal has been launched to raise money to help those people fleeing violence in Myanmar.
In the past month, hundreds of thousands of people have fled violence in Rakhine state with most of them seeking refuge in Bangladesh.
More than 500,000 people, mostly Rohingya women and children, are at crowded camps and settlements but many have no access to clean drinking water, toilets or washing facilities and there is a growing risk of disease.
The UK Government has vowed to match the first £3m donated by the public to the appeal, which will be launched by the Disasters Emergency Committee with a video on all major UK broadcasters later.
DEC chief executive Saleh Saeed said: "This is one of the fastest movements of people we have seen in recent decades.
"This humanitarian catastrophe is unfolding in a country that is already reeling from the worst flood in decades.
Aung San Suu Kyi, Myanmar’s leader, has been criticised for not condemning the violence in Rakhine state.
Regarding the many thousands of Rohingya Muslims who have fled for Bangladesh, Ms Suu Kyi said in September: "We want to find out why this exodus is happening.
She added: "It is utterly intolerable that the military who are responsible for this inhumane catastrophe have not heeded calls for restraint and pleas to allow those who are now refugees to return safely to their homes."