Teams from Italy will join those from Ireland, Scotland and Wales in the Magners League from next season.
The three Celtic unions agreed unanimously to allow their Italian brethren in, with two teams - Benetton Treviso and Aironi Rugby - joining from the start of the 2010-2011 season.
The decision to allow the Italian teams in comes after a year-long review of the viability of expanding the tournament.
"This is another massive step forward for the Magners League," said David Jordan, Director of Celtic Rugby.
"It is anticipated that the Italian super clubs will be largely made up of Italian internationals of full, A and under-20 levels, with a sprinkling of top class overseas players set to provide further extra strength and spice to the tournament."