Tony Blair used to have a line in his speeches to the effect “every Labour government has created a department for international development, and every Tory government has got rid of it”.

It has taken a while for this Conservative government to fulfil this historic mission. David Cameron saw the preservation of the department, and the commitment to spend on 0.7% on aid as central to giving the Conservatives a modernising message. He often cowered in front of the Daily Mail, but not on the issue of aid, and its contribution to Britain’s reputation. On Tuesday he took the unusual step of branding the merger as a mistake.

Theresa Maypreferred to nibble away at the aid budget, handing tranches to other Whitehall departments. By 2019, the Foreign Office spent £680m on aid, more than double the £300m it spent in 2013 and equivalent to 40% of core departmental allocation of £1.7bn.

Priti Patel, the international development secretary until her defenestration in 2017 for running her own Middle East policy, also probed her room for manoeuvre through changing the OECD’s definition of aid.

But Boris Johnson, as a former foreign secretary, has always resented the independent aid budget. He wanted to bite the bullet, and came to be supported by the Foreign Office minister, Rory Stewart, who felt the Dfid mindset was opposed to seeing the UK foreign policy stance as a whole, and instead focused exclusively on poverty reduction in the world’s poorest countries.

Stewart recalled as a joint minister in both departments the battles he faced in the Foreign Office to get a £200,000 cheque cleared while in international development he could add a couple of digits, and few objected.

At first it looked as if Johnson as prime minister would follow May and merge the departments through stealth, or at least wait to wrap up the change as the centrepiece of the defence and security review due initially to be completed in the autumn.

Instead he plumped for joint FCO and DfID ministers – meaning the two departments had one joint ministerial team, but two very different-sized budgets. They also had two permanent secretaries, leading the head of the diplomatic service, Sir Simon McDonald, to say euphemistically: “They were working through the consequences of closer alignment.”

But Johnson, backed by the Great Disruptor Dominic Cummings, chose the coronavirus outbreak as a surprising moment to strike, and Cummings kept the plan unbriefed and unknown to all but a few.

At one level the timing was propitious. Anne-Marie Trevelyan, the new secretary of state for international development carried little clout across Whitehall.

Johnson also came to the view that Downing Street was held back by unnecessary duplication in the Whitehall machinery. His aides said he decided what he wanted to do and delay served no purpose.

The complaints of the well-organised aid lobby – also disliked by Cummings – would be submerged in the coronavirus crisis.

The counter argument is twofold. Coronavirus is creating an international humanitarian emergency, and civil servants could do without a four-month bureaucratic reorganisation. Equally, there seems little point holding a foreign and defence review when the likely main conclusion has now been reached.

Johnson has tried to reassure anti-poverty campaigners that the commitment to spend 0.7% of GDP in aid remains in law. But within that budget – bound to collapse in real terms due to the fall in GDP – Johnson has a great deal of room for manoeuvre.

If the future budget decision maker is not a development project specialist, different programmes will find favour.

“The ghost of the Pergau Dam disaster still hangs over UK aid,” said one aid specialist, referring to Margaret Thatcher’s decision to grant Malaysia £234m aid in return for an arms deal.

It took until 1994 for the contract to be declared unlawful by the high court.

UK state aid can quite rapidly become the gift that opens the door for UK plc to gain lucrative contracts with foreign leaders. The blurring of the two will be made easier as Johnson has decided the UK’s trade representatives will leave the Department for Business and work in the Foreign Office.

Some say this is a fantasy and the Department for International Development will carry on largely as before within the Foreign Office. Mergers are common abroad. But there is no need to look into the crystal ball when you can read the book. The Foreign Office already has a slice of the aid budget, and spends it less transparently and less efficiently, all independent analysis commissioned by the government shows.

Only this week the international development select committee reported UK aid not administered by the Department for International Development “has a very different geographical profile, with around three quarters going to middle-income countries, including China, India and South Africa, pursuing priorities such as reducing carbon emissions, tackling insecurity, building research partnerships and promoting trade and investment ties with the UK”.

Many Conservatives predict disaster, notably Andrew Mitchell, the former secretary of state for international development. “If you fold it into the Foreign Office, all the brilliant people who have given DfID its reputation around the world as the prime example of global Britain, in terms of British influence and effectiveness, will leave. They’ll all go and work in the international system.

“We will at a stroke have collapsed the pre-eminent and most respected engine of international development in the world.”