Wearing a colourful dashiki shirt given to him by the president of Rwanda, Andrew Mitchell leaned forward in his armchair, eyes gleaming, and explained why the Conservative party’s pledge to spend 0.7% of GDP on international development was solid. Any attempt to change party policy on aid would fail, Mitchell told me because “there are too many of us who won’t put up with it”.

That might have been the case when we met 18 months ago – it’s not now. The political consensus on aid, which has lasted two decades and led to Britain becoming a world leader in aid and development, has fallen apart.

The current international development secretary, Penny Mordaunt, delivered a presentation proposing a series of radical changes to the way Britain delivers aid, and how much we spend on it. She floated the idea that not all of the money should come from taxpayers (some could come from companies or philanthropists, she said), and proposed spending some of the department’s budget on peacekeeping. In other words, aid money would be spent on deploying British troops.

Mordaunt is not the only Conservative with the aid budget in their sights. Jacob Rees-Mogg has long campaigned for it to be slashed, while the former foreign secretary Boris Johnson has renewed his call to scrap the Department for International Development (DfID) and bring aid and development funding back into the Foreign Office, two decades after the Labour government spun it off.

It’s notable that aid’s biggest critics are also Brexiteers – and not just in parliament. The most prominent outriders for a cut to the aid budget – newspapers such as the Sun, Daily Mail and the Express, and opaque lobby groups such as the TaxPayers’ Alliance and the Institute of Economic Affairs – are also Brexit’s loudest cheerleaders. That’s no coincidence: a form of nationalism, particularly the idea of a generous Britain being taken for a ride by foreigners, fuels both campaigns.

The idea of rich nations spending 0.7% of GDP on international aid sprang from a recommendation made by a World Bank commission in the late 1960s. This was an era of decolonisation, independence and hope—there was a growing consensus among rich nations that they had a duty to help poorer nations.

But warm words and international targets don’t necessarily lead to change – by the time Labour came to power in 1997, we spent just 0.26%. Over the next decade, that figure would rise, eventually reaching the 0.7% target.

It was a target that David Cameron was determined to keep – it formed a key part of his early modernisation agenda. Alongside an apparent commitment to combat climate change, the promise on aid was a symbol of change within the Conservative party. Even after the financial crash, as the Conservatives dropped their commitment to matching Labour’s spending and introduced an era of austerity, aid spending, like the NHS budget, was ringfenced.

Theresa May considered scrapping the pledge but at the 2017 election the aid industry fought a guerrilla operation, dominating the news cycle for the first three days of the campaign, deploying a succession of high-profile aid supporters from Bill Gates to Rowan Williams. The prime minister relented and agreed to keep the 0.7% pledge.

A repeat of that campaign will be difficult next time around. For a start, the aid budget’s opponents, which clearly include the current DfID secretary, are better prepared. Don’t be surprised if a commitment to cut 0.7% is made by one, if not all, of the candidates to replace May the next time the Conservatives choose a leader.

But perhaps the bigger problem for the aid industry is the aid industry itself. Since the sexual exploitation scandal at Oxfam, which then spread into a wider debate including harassment at Save the Children, Britain’s largest NGOs have been wary of sticking their heads above the parapet. NGOs have also faced criticism, particularly from many of their own experts, that too many campaigns are fronted by celebrities, focus on the most heartbreaking stories and make too little effort to talk about long-term structural aid.

A decade of austerity has also taken its toll. Robert Halfon, a one-time supporter of 0.7%, switched because he said he found it impossible to argue that there should be cuts in his constituency but not in the developing world. Aid supporters need to recognise that while the hard right may be leading the charge, more moderate voices are also thinking again.

Yet what none of those who propose scrapping DfID or reducing its budget like to acknowledge is that it’s actually very good at spending money. It ranks third in the annual Aid Transparency Index, and among international aid experts has one of the best reputations in the world. A report published this month by the One campaign also praised DfID for its effectiveness, transparency and focus on poverty. In short, DfID is working.

Would the Foreign Office be any better? Based on the current evidence, no. In recent years, portions of the aid budget have been parcelled out to other departments, including the Foreign Office. All of them, according to the One campaign report, struggle to spend them well. And the Foreign Office? Nearly at the bottom of the pile, just above the Home Office.

The greatest irony is that those leading the campaign to undermine Britain’s aid effort are also those who like to loudly trumpet the idea of “global Britain”. But if global Britain means anything, surely it means using the UK’s expertise and its money – we’re the sixth largest economy in the world, remember – to help those in need.

British aid is not perfect. But it has transformed millions of lives, helped end humanitarian emergencies and boosted British soft power. It would be a tragedy if, post-Brexit, we retreated further into our shell and destroyed one of the few things we’re actually good at.

• Steve Bloomfield is the deputy editor of Prospect magazine