Leading charities and senior Conservatives have joined forces to urge Boris Johnson to abandon plans to axe the UK’s £14bn foreign aid department, warning it would be “turning our backs on the world’s poorest people”.

An alliance of more than 100 aid groups spoke out as officials began work that is expected to see the Department for International Development (DFID) swallowed up by the Foreign Office (FCO).

They warned the post-election shake-up would inevitably lead to less aid reaching poverty blackspots – a trend identified with the 30 per cent of aid already being spent by other departments.

It would also backfire by making it harder to know if funds are being spent “effectively” without the transparency provided by the DFID, a problem already raised by the National Audit Office.

The warning – echoed by two former Conservative ministers – came after government sources confirmed to The Independent that work towards abolishing the department is underway.

It would confirm an idea first mooted by Mr Johnson almost a year ago for after Brexit, when he argued it would ensure “Global Britain is going to achieve its full and massive potential”.

But the charities’ statement said: “Merging the DFID with the FCO would risk dismantling the UK’s leadership on international development and humanitarian aid.

“It suggests we are turning our backs on the world’s poorest people, as well as some of the greatest global challenges of our time: extreme poverty, climate change and conflict.

“UK aid risks becoming a vehicle for UK foreign policy, commercial and political objectives, when it first and foremost should be invested to alleviate poverty.”

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Among the charities protesting are World Vision UK, ActionAid UK, Oxfam, Women for Women International UK, War on Want, World Jewish Relief and Islamic Relief UK, Tearfund, VSO and Mothers Union.

The wide-ranging shake-up of Whitehall departments will get underway once the UK has formally left the EU on 31 January, government sources have confirmed.

It is also expected to create a powerful new business department – absorbing the international trade department – in a bid to secure inward investment to the regions, despite Brexit.

A new economic “super-ministry” is likely to take responsibility for broadband and artificial intelligence – and the department for energy and climate change recreated.

The “merger” of the DFID with the FCO was proposed, last February, in a pamphlet penned by a backbench Tory MP and endorsed enthusiastically by the future prime minister.

It also advocated a big de facto cut to the commitment to spend 0.7 per cent of national output on overseas aid, enshrined into law by the Cameron-Clegg coalition.

The report proposed the budget should fund all peacekeeping work and world service broadcasting, reworking for the UK government’s own policy aims beyond economic development in poor countries.

Mr Johnson threw his weight behind the proposals, saying: “We could make sure that 0.7 per cent – a huge sum of money – was spent more in line with Britain’s political, commercial and diplomatic interests.”

He also accused the department of “inevitable waste as money is shoved out of the door in order to meet the 0.7 per cent target” – raising fears its budget will be slashed.

Mr Johnson said, earlier this year: “We can’t keep spending huge sums of British taxpayers’ money as though we were some independent Scandinavian NGO [non-governmental organisation].

“It is perfectly possible to boost global development in a way that coheres much better with UK political and indeed commercial objectives.”

Both Alistair Burt, a former DFID minister who stood down at the election, and Andrew Mitchell, a former international development secretary, urged Mr Johnson to think again.

“My advice would be not to merge DFID and the FCO. DFID as a standalone department has given the UK an outstanding reputation. It runs very well,” Mr Burt said.

“It has learned very well over the years the rules about how to handle its aid budget. It is conscious of the risk presented in delivering such a large aid budget.”

And Mr Mitchell said: “DFID is the most effective and respected engine of development anywhere in the world, and a huge soft power asset for Britain.

“Any machinery of government changes in Whitehall should obviously respect Britain’s international development in the poorest and most unstable parts of the world.