In a letter to the prime minister, 188 humanitarian and development charities, NGOs and think tanks said the move suggested the UK was “turning its back on the world’s poorest people”.
“It also risks us being less able to respond to the great challenges of our time, such as global health security and climate change,” the charities add.
“This decision, taken during a global pandemic with no consultation, ahead of the review of development, diplomacy and defence and against the recent advice of the cross-party International Development Select Committee, does not enhance our reputation in the world but diminishes it.”
The charities said that the sector was not consulted about the plans to close Dfid, which was created after Labour won the 1997 general election.
Opposition MPs and three former prime ministers, including the Conservative David Cameron, have criticised the decision.
It comes as the head of Save the Children UK warned the world is on the brink of “potentially the worst reversal in progress for children that we’ve seen since 1945”.
CEO Kevin Watkins said it was a “terrible decision” announced with “appalling timing” and added: “The UK is going to face enormous fiscal pressures in the years ahead.
“The UK public are incredibly generous in the way they respond to humanitarian emergencies, the UK public is not inward looking and disinterested in the plight of people in the poorest countries but there clearly is a danger that there will be political forces saying ‘let’s scale down the aid programme in order to scale up spending in priority areas in the UK’.
“We believe we can have a fiscal framework that tackles poverty, health, educational challenges in the UK and extend that moral purpose to developing countries.”
He added: “In the world’s poorest countries you’ve got over a billion children who are now out of school, who are at real risk ... in the same way we have to view education as a really critical part of life chances for young people in the UK, that’s equally true in the poorest countries.”
Stephanie Draper, chief executive of Bond, the UK international development network, said: “This would be a disaster for the UK’s credibility as a world leader in development and aid, especially at a time when Covid-19 requires a global response, without which it remains a threat to us all.”
A government spokeswoman said: “As the PM has said, the merger of Dfid and FCO will ensure that all of our national assets – including our aid budget and expertise – are used to safeguard British interests and values overseas.
“This will strengthen our ability to lead the world’s efforts to recover from the coronavirus pandemic and allow us to seize the opportunities ahead as we prepare to take on the G7 presidency and host COP26 next year.
“The work of UK aid to reduce poverty will remain central to the new department’s mission and we will continue to be guided by our responsibilities under the International Development Act, including a commitment to the target of spending 0.7 per cent, which is enshrined in law.
“The government will continue its ongoing engagement with UK and international NGOs, including on issues relating to the merger.”
Additional reporting by Press Association
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