The World Health Organization (WHO) is to offer paid internships for the first time to boost access for those applying from developing countries, the BBC has learnt.
About 1,200 interns are accepted by one of the UN's largest agencies each year to support its work improving public health and tackling global diseases.
Yet fewer than one in four of those chosen are from low-income countries.
For the past 50 years, the WHO has expected its interns to move to their headquarters in Geneva, or one of its six regional offices, and work unpaid without travel expenses for up to six months, costing each person around £5,000 ($6,540).
But after a campaign led by a former intern, the UN agency has agreed to provide full financial support for its young workers by no later than 2020.
It told the BBC that targets are also in place to ensure that 50% of interns come from developing countries by 2022.
"It's unacceptable that 80% of WHO's work goes into supporting people in developing countries, yet only 20% of their interns come from them," says Ashton Barnett-Vanes, 29, a British doctor of English and Jamaican heritage from Wolverhampton, who started the campaign after his internship in 2012.
He spent six years rallying the international community before eventually working with ministers across the Caribbean and Africa to persuade the 194 members of the UN to reach the agreement.
"All it took was for one country to say 'no' for it to be stopped," Dr Barnett-Vanes said. "If you don't have trained staff, you can't develop an effective health system," he added.
"This change could see more than 500 young people each year from developing countries receive professional training that they can invest back into their communities."
In 2015, Dr Barnett-Vanes and Tara Kedia, 27, a fellow former intern who now works in health policy in Washington DC, launched a crowdfunding campaign to create a $10,000 scholarship to enable two young people to intern at WHO.
Christabel Abewe, 28, from Uganda, and Yassine Kalboussi, now 29, from Tunisia, were chosen.
"I wouldn't have been able to participate without the scholarship," Ms Abewe said.
Dr Kalboussi agreed: "It would have cost two full years of my salary."
"Professional public health training is not currently provided in Tunisia," he added. "I was one of only three interns from a low or middle-income country out of my group of 162 people."
Dr Kalboussi now serves on the executive committee of the Tunisian Center for Public Health, and Ms Abewe is researching the early detection of breast cancer in developing countries.
More than 100 UN states have no participants each year, while 50 countries - including Angola, Barbados, Cambodia, Cuba and Libya - did not have a single WHO intern between 2015 and 2017, according to WHO data.
Jamaica has had one WHO intern in the past two years.
Dr Christopher Tufton, the Jamaican health minister, championed the campaign in Geneva. "We should make it easier for young people to access these facilities and view it as an investment in the people who participate," he said. "They should be valued."
The WHO has admitted its selection process is not "merit-based" and told the BBC that it recognised its unpaid internships were unfair.
It said that funding for 50 interns per year had already been secured from the Wellcome Trust, a London-based medical research charity, but said "more support was needed".
"If we are to nurture the next generation of global leaders who truly come from all four corners of the earth, we need to facilitate them spending time with us," the WHO media team said.
As a whole, the UN had more than 38,000 interns between 2009 and 2017, but more than 80% (about 30,400) were unpaid.
Based on these figures, this would amount to nearly two million interns in the past 50 years - more than the entire population of Trinidad and Tobago.
A report by the UN's Joint Inspection Unit found that interns had to "endure harsh living conditions" to last the duration of the programme or work "after hours" and informal jobs such as dog-walking to make ends meet.
Headlines were made in 2015 when former UN intern David Hyde, who was then 22, was found sleeping in a tent on the shores of Lake Geneva after finding the cost of living in the city too expensive.
He later admitted it was a stunt to spark a discussion about "unjust unpaid work".
The WHO is the first UN agency to pass a resolution on internships that was brought about by its member states.
As the UN General Assembly is about to start its session, the campaigners hope the issue of unpaid internships will be back on the agenda in light of this year's theme - "Making the United Nations relevant to all people".
"It's about time the UN solved this issue," said Dr Barnett-Vanes. "It's within its gift to and it should follow WHO's lead. "If the UN isn't willing to act, its member states should."