Britain’s status as a world superpower in development is in “safe hands” under Dominic Raab, according to the international development secretary, as she prepares to leave her post. In an interview with the Guardian, Anne-Marie Trevelyan expressed sadness at leaving the Department for International Development (DfID), whose work is “truly impactful” and “doing good”, she said. But she said she has seen passion and enthusiasm in the foreign secretary towards helping developing countries become stronger. Raab will be in sole charge of the new Foreign, Commonwealth and Development Office (FCDO) when DfID and the Foreign and Commonwealth Office merge and Trevelyan leaves her cabinet post in September. Boris Johnson’s announcement last month of the merging of the Whitehall departments has been widely criticised, including by three former prime ministers who predicted it would mean “less respect for the UK overseas”. Trevelyan, speaking from the Isle of Wight on what is likely to be her last trip as development secretary, said: “It’s always sad. Being the secretary of state is an all-consuming role and especially in a department that is doing good. It doesn’t get more interesting, more challenging or more rewarding to be able to make change.” Citing the UK’s £1.65bn pledge to Gavi, the vaccine alliance, as an example of DfID’s influence, Trevelyan said she would be “very sad” to no longer be doing that type of work. “The convening power that I was able to draw on to ensure that this organisation has five years of funding to vaccinate and save the lives of millions of children … Those sorts of things … are truly impactful, so yes, I’ll be very sad not to be doing that,” she said. But, she added: “It’s been a really lovely experience to watch the foreign secretary start to understand how incredibly impactful what we do with ODA [overseas development assistance] money is. I hand it over to him in his new role with the FCDO knowing that he’s developing the same passion for seeing how we can best use taxpayers’ money to really make a difference and help those countries become stronger. “Because that’s what it should be. It shouldn’t be the continuous drip of aid, without outcome. It should be helping them to no longer need us. To get it right, countries can just be partners through trade and culture, equal partners.” She was confident Raab would be a safe pair of hands. “I tease him; I say I’ll sit on the back benches and quietly cheer him on. We are considered a world superpower in international development terms. It will rest with him.” Asked if she was disappointed not to be asked to head up the new department, Trevelyan said: “It’s the PM’s choice and I look forward to a few weeks off to catch up with some sleep.” Trevelyan said it had been tough to cut the aid budget by £2.9bn, but she had worked “line by line” to prioritise key areas, such as women and girls, education and projects to tackle the climate crisis. The development secretary was on the Isle of Wight this week to visit Micron Group, which is providing pesticide spraying equipment to the UN’s Food and Agriculture Organization to tackle the locust plague that is devastating thousands of hectares of crops across Africa and Asia. She announced £18m of new money in response to the crisis. Trevelyan said Micron Group was providing “exactly the spirt of technical engineering and support” needed to get ahead of the locust swarm. “But unless other countries also step up and act now, this crisis will spread and cause even more devastation,” she said. Trevelyan was accompanied on her visit by Bob Seely, MP for the Isle of Wight, whose plan to close DfID as part of a vision for a post-Brexit “global Britain” was backed by the prime minister last year. Seely, a Tory member of the foreign affairs select committee, said he believed critics of the merger would be “proved wrong and we will emerge from this with an enhanced overseas foreign policy with the aid element and how we support people in other parts of the world as a critical part of it”. His argument for a merged department was that Britain could do more good by integrating. Seely’s paper said the UK should be free to set its own aid spending, unconstrained by criteria set by external organisations, and that the department’s purpose should be expanded from poverty reduction to include “the nation’s overall strategic goals”.