Merging the Department for International Development (Dfid) with the Foreign Office is an “unnecessary and expensive distraction” that will diminish the UK’s reputation, almost 200 charities believe.

The Government’s decision to scrap the department through the merger suggests the UK is turning its back on the world’s poorest people, 188 humanitarian aid and development charities, NGOs and think tanks said.

The groups, which include Save the Children UK, ActionAid UK and Tearfund, have written to Prime Minister Boris Johnson asking him to urgently reconsider.

This decision ... does not enhance our reputation in the world but diminishes it

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”.

The letter reads: “Abolishing one of the world’s most effective and respected Government departments at a time when the world is in need of global leadership undermines our response to Covid-19 and suggests the UK is 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 say it is an “unnecessary and expensive distraction” from building up Britain’s role in the world and the sector was not consulted about the plans.

It concludes: “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 decision to close Dfid after 23 years and create the Foreign, Commonwealth and Development Office has also come under criticism from opposition MPs and three former prime ministers, including the Conservative David Cameron.

Save the Children UK chief executive Mr Watkins said it is a “terrible decision” announced with “appalling timing”.

He fears some politicians will call for the aid programme to be scaled down to increase domestic spending as the UK deals with the economic fall-out from Covid-19.

Now is the “the testing ground for whether ‘Global Britain’ is a real commitment or an empty phrase”, he said.

Mr Watkins told the PA news agency: “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: “Every parent in the UK is desperately concerned about education in this country and especially the education gaps that are opening up because of lockdown.

“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 the sector fears aid will be directed away to be used for trade deals.

“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,” she said.

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%, which is enshrined in law.

“The Government will continue its ongoing engagement with UK and international NGOs, including on issues relating to the merger.”