The appeal is being made by the Disasters Emergency Committee (DEC) – an umbrella organisation of 13 UK humanitarian aid agencies.
The UN says four million people have been affected by the floods, and at least 1,600 have died.
Donations can be made by phone, post, at Post Offices and banks.
The TV and radio appeal was presented by journalist and former British hostage in Lebanon, John McCarthy.
The DEC said aid from its members was now reaching more than 300,000 survivors.
DEC chief executive Brendan Gormley said the appeal was crucial “because of the scale of the tragedy”.
Aid agencies are already working in the region, distributing clean water and food parcels, but “they want to scale up their work, so this is the way of going to the public and saying ‘please help’,” Mr Gormley said.
The DEC charities are ActionAid, Age UK, British Red Cross, Care International, Cafod, Christian Aid, Concern, Islamic Relief, Merlin, Oxfam, Save the Children, Tearfund and World Vision.