A spokesman for Hope for Justice said that, although this was not yet a full merger, Retrak was expected to be absorbed into Hope for Justice in the long term as part of a move to reduce management and support costs, share expertise and improve the charities’ impact on human trafficking.
Both charities will retain their own charity registrations, accounts, databases and fundraising "for the time being", the spokesman said, and no date had yet been set to complete the full merger.
There will be four redundancies at Retrak’s head office as a result of the new relationship between the charities, with the charity moving into Hope for Justice’s headquarters in Manchester.
Retrak employs 176 staff, six of which are based in the UK, with the rest operating in Uganda and Ethiopia. Hope for Justice has more than 100 staff based in the UK, Cambodia, the US and Norway.
Retrak’s offices in Cheadle Hulme, Stockport, will close as a result of the partnership between the two charities.
The two charities have similar incomes, with Hope for Justice bringing in £1.8m in the year ending March 2017, and Retrak earning £2.1m in the same period.
Ben Cooley, chief executive of Hope for Justice, will become Retrak’s new chief executive as part of the merger.
Retrak’s previous chief executive, Sir Peter Fahy, a former chief constable of Greater Manchester Police, will become director of structural reform at Hope for Justice.
Both charities will retain separate boards, but will eventually have the same trustees. Peter Elson, chair of Hope for Justice, will also be the chair of Retrak.
Cooley said: "This gives us an opportunity to serve more people in a sustainable and replicable way, and to bring on board immense amounts of front-line experience, insight and knowledge.
"This integration gives us the ability to help more adults and more children who have been walking through some of the darkest days – to give them hope and to give them justice."
Fahy said: "Becoming part of the Hope for Justice family will create an organisation that can have an impact on the full spectrum of trafficking and modern slavery, from the rural village where family separation takes place to the city streets, where children are at serious risk of exploitation and trafficking."