Europe and US should also be responsible for millions who will be displaced by climate change, says Abul Maal Abdul Muhith

Up to 20 million Bangladeshis may be forced to leave the country in the next 40 years because of climate change, one of the country's most senior politicians has said. Abul Maal Abdul Muhith, Bangladesh's finance minister, called on Britain and other wealthy countries to accept millions of displaced people.

In a clear signal to the US and Europe that developing countries are not prepared to accept a weak deal at next week's Copenhagen climate summit, Abdul Muhith said Bangladesh wanted hosts for managed migration as people began to abandon flooded and storm-damaged coastal areas.

"Twenty million people could be displaced [in Bangladesh] by the middle of the century," Abdul Muhith told the Guardian. "We are asking all our development partners to honour the natural right of persons to migrate. We can't accommodate all these people – this is already the densest [populated] country in the world," he said.

He called on the UN to redefine international law to give climate refugees the same protection as people fleeing political repression. "The convention on refugees could be revised to protect people. It's been through other revisions, so this should be possible," he said.

Tens of thousands of people in Bangladesh and other low-lying areas of Asia are leaving their communities as their homes and land become inundated. But this is the first time that a senior politician from a developing country has openly proposed that those countries considered responsible for climate change should take physical responsibility for the refugees created.

Bangladesh, India, and many small island states such as the Maldives face having to relocate large populations over the next 50 years as sea levels rise up to one metre. This would have profound effects on the 1.5 billion people who presently live in coastal areas. The Intergovernmental Panel on Climate Change (IPCC), the scientific body that assesses the impact of climate change, has said there could be 200 million climate change migrants by 2050.

There is mounting evidence in India and Bangladesh and other low-lying countries that sea levels are rising faster than the global average of 1.2mm a year. Islands and coastal communities in the Ganges delta and the Bay of Bengal have recorded rises of up to 5mm a year. In Bangladesh hundreds of coastal villagers are forced to drink salty water as tides continue to rise and the sea intrudes on fresh water aquifers.

Abdul Muhith said managed migration could be positive for Bangladesh and the west: "We can help in the sense of giving the migrants some training, making them fit for existence in some other country.

Managed migration is always better – we can then send people who can attune to life more easily." But he added, in another warning before Copenhagen where money will be a critical issue, that current levels of aid were inadequate. "Total aid in Bangladesh today is less than 2% of GDP. It is almost the same in China and in India. So we, the most populated, least developed country, gets peanuts. This inequity is terribly intolerable."

Rajendra Pachauri, chairman of the IPCC, said the Bangladeshi migration proposal should be taken seriously. "This is clearly a warning signal from Bangladesh and similar countries to the developed countries. And I think it has to be taken very seriously. If you accept that those countries that have really not been responsible for causing the problem, and have a legitimate basis for help from the developed countries, then one form of help would certainly be facilitation of immigration from these countries to the developed world," he said.

"If you had 30 or 40 million migrating to other parts of the world, that's a sizable problem for which we have to prepare. And if it requires changes to immigration laws and facilitating people settling down and working in the developed countries, then I suppose this will require legislative action in the developed world," he said.

Douglas Alexander, the international development secretary, said: "As the largest international donor to Bangladesh, Britain has been urging the international community to provide extra money for climate change adaptation." But Jean-Francois Durieux, who is in charge of climate migration at the UN refugee agency, cautioned against reworking the UN convention on refugees.

"The risk of mass migration needs to be managed. It's absolutely legitimate for Bangladesh and the Maldives to make a lot of noise about the very real risk of climate migration – they hope it will make us come to their rescue. But reopening the 1951 convention would certainly result in a tightening of its protections."

He said there was a danger of a backlash in rich countries. "The climate in Europe, North America and Australia is not conducive to a relaxed debate about increasing migration. There is a worry doors will shut if we start that discussion," he said.

There is extreme sensitivity about adapting the UN convention on refugees. A UNHCR report in August warned: "In the current political environment, it could result in a lowering of protection standards for refugees and even undermine the international refugee protection regime altogether."