By Sarah Grainger
BBC News, Kampala
Rwanda and Burundi are formally to join the East African Community at a meeting in the Ugandan capital, Kampala.
Members reported increased revenues since the customs union
Leaders of the three existing members, Kenya, Uganda and Tanzania will welcome Rwandan President Paul Kagame and President Pierre Nkurunziza of Burundi.
Their membership will take effect from 1 July. Consultations are also under way on forming a political union.
The group was founded in 1999 and a customs union between the original members came into effect two years ago.
The East African Community (EAC) heads of state are expected to sign treaties of accession which will admit Rwanda and Burundi into the regional grouping.
Although the EAC has been in existence in its current form for eight years, it is the customs union that has been the most significant development to date.
It means that member countries pay no duties on goods within the East African Community.
Kenya, Tanzania and Uganda have all reported increased revenues since the customs union was launched.
Trade between the three countries and Rwanda and Burundi will also now be free of customs tariffs, making the movement of goods across the region cheaper.
There are also plans for further integration; an East African common market is to be established by 2010 and the possibility of a single currency has been raised.
Consultations on political integration have been taking place in Kenya, Tanzania and Uganda since October. A federal president and government could be in place within seven years.