In a statement, the charities said they would operate under a new name – United Purpose – and the merger was part of a strategy to have more partnership working to "increase collective impact, improve efficiencies in the sector and modernise the structures of UK-based development charities to be fit for the future".
Concern Universal, which has 25 staff in the UK but more than 600 worldwide, works with people in some of the poorest countries in the world to find sustainable solutions to poverty and inequality.
International Inspiration, which uses sport to provide a platform for education in developing countries, has two members of staff.
United Purpose, which will be based at international headquarters in Cardiff, said it had adopted a new strategy to push forward new approaches and forms of financing to tackle poverty. International Inspiration’s office in London will remain open as an additional base for the new charity.
Kathryn Llewellyn, chief executive of Concern Universal, will become global chief executive of United Purpose.
International Inspiration was previously led by an interim chief executive, Jehangir Malik, who has since left to become the chief executive of Muslim Aid.
Sir Martin Davidson, chair of International Inspiration, will join United Purpose’s board, with the rest of International Inspiration’s trustees providing advice and support to the new charity’s board.
Llewellyn told Third Sector that the charities had begun negotiations about a partnership 12 months ago and the talks developed into a formal merger. She said the move was part of the charity’s desire to see more joint working across the international development charity sector.
She said there would be no job losses as a result of the merger.
"As the world evolves, the demands and expectations on development NGOs have also changed," she said in a statement.
"We must radically re-imagine and change our approach as NGOs if we are to stay effective in a changed world. If we don’t, we face well-deserved extinction or irrelevance.
"To stay true to our mission to help people lift themselves out of poverty, we are also setting out an ambitious new path on how we are going to achieve this – and a new strategy and name to reflect this."
According to figures on the Charity Commission website, Concern Universal had an income of £19.3m in the year to 31 March 2015 and expenditure of £18.5m.
International Inspiration had an income of £819,949 and spent £1.2m in the same accounting period.