The merger will allow the two charities to combine their messages into something more powerful, said Angela Smith-Morgan, co-chief executive of Leukaemia UK.
Leukaemia UK will become part of Leuka after the merger in order to retain Leuka’s CIO status.
But the branding of the charity will be under Leukaemia UK, with a new logo and strapline "Blood cancer research and care".
Growth is also a motivation for the merger, and Smith-Morgan said that both charities were "very strong financially".
"It’s a merger of equals and we will do so much better," she said.
Leuka, which focuses on leukaemia research, had an income of £1m from its last accounts covering the year to 31 December 2018.
Leukaemia UK focuses on providing care for those who suffer from blood cancer, and had an income of £507,000 in the same financial year.
Both charities had four staff members before the merger and the newly merged charity will have five part-time staff and three full-time staff in total. The merger will not result in any role changes.
The chief executive role will be shared by the former Leuka chief executive, Olive Boles, and Smith-Morgan, who said that the two chief executives would "split the role and work towards our strengths".
The new board will comprise all the trustees from both charities, so there will be 15 members. But over time this would be reduced, said Smith-Morgan.
There were previously seven Leukaemia UK trustees before the merger and eight on Leuka’s board.
Leukaemia UK’s staff have moved to Leuka’s offices in central London near Regent’s Park.
Boles said: "It is a privilege to lead this charity with Angela and to build on our successes. The whole team is committed to ensuring Leukaemia UK makes the biggest possible impact."