Christian Aid says heavy and persistent rainfall since mid-July is forcing hundreds of thousands of people to abandon their homes.
It is warning the heavy rains are destroying homes as well as water sources, food stores, farmland and roads.
Shakeb Nabi, Christian Aid's Country Manager for Bangladesh, told Premier: "Needs are huge, we have been able to reach around 5,000 [people]... through our own resources and through resources in the UK, but the gap is still huge, we need to overcome this gap and I feel that the people of the UK will stand by our side."
He went on to say that "people have totally lost their lives, totally lost their houses and at the same time they've lost their livelihood options."
Rains have been relentless for around 25 days.
Worries about a lack of shelter for displaced families, as well as malnutrition and how to limit the spread of water-borne diseases continue.
More than 3.5 million people in northern Bangladesh are affected.
Christian Aid is heading up a £250,000 multi-agency response and local groups on the ground are giving out relief packages to more than 17,000 people.
Parcels contain food, soap, disinfectant, tarpaulin and groundsheets, female sanitary items, rehydration tablets, cooking utensils and cash, to enable people to purchase other essentials.
Women and girls are disproportionately affected by the flooding.
Rozana Majumdar, Christian Aid's Gender and Governance Programme Officer for Bangladesh, explained: "Women and girls are considered the worst victims of disasters and the vulnerabilities they face are unlike those faced by men.
"Not only have many women lost their homes in the floods, but many of them are pregnant or have recently given birth."
"Bangladesh is a male-dominated society, where the needs of women and girls are often not taken into consideration during disasters.
"That is why Christian Aid and our local partners are taking measures to ensure the safety of women and girls before, during and after disasters, in a way that is culturally and religiously sensitive and appropriate."
Listen to Premier's Hannah Tooley speak to Shakeb Nabi here: