The league currently has 10 teams from Wales, Ireland and Scotland
The Magners League is to be expanded from 10 to 12 teams from the start of next season as two Italian clubs are set to join the Celtic competition.
The Celtic Rugby Board board have given Treviso and a renamed Viadana a four-year licence into the revamped League.
The move is the biggest boost for Italian rugby since the national team's entry into the Six Nations in 2000.
The two Scottish, four Welsh and four Irish clubs have received financial guarantees from the Celtic board.
The current teams - including reigning champions Munster - feared they might have lost out financially with the admission of the Italians.
So after a year-long viability review, league chiefs reassured the current members about the financial and commercial conditions.
I am sure that the Magners League will bring benefits to all Italian rugby and will have a positive impact on the competitiveness of our national team
Giancarlo Dondi, President of Federazione Italiana Rugby
There had been fears Scottish teams Edinburgh and Glasgow's participation in the Magners League was in jeopardy after a supposed disagreement with their Welsh and Irish counterparts over a proposal to include the Italian teams.
But the three Celtic Unions unanimously agreed to the introduction of two new Italian super sides after further reassurances of the competitive strength of the two Italian entrants and the standard of their facilities.
The Italians joining the Magners League, which has been in discussions for the best part of 18 months, is expected to be widely well-received in rugby as it should significantly improve Italian rugby standards.
Benetton Treviso and the new Aironi rugby region - based at Viadana's Stadio Luigi Zaffanella - which will involve various clubs from the Lombardia and Emilia-Romagna regions will enter the Magners League in its 10th season.
Federazione Italiana Rugby (FIR) propose to have both Benetton Treviso and Aironi's Heineken Cup qualification ring-fenced for the next four years - but that move must be ratified by the ERC.
There had been hope of creating a Magners League super club in the capital Rome but that proposal collapsed so both Italian clubs will be based in the rugby stronghold of northern Italy.
Celtic Rugby director David Jordan said: "This is another massive step forward for the Magners League and follows the decision to go to a top four play-off system to determine our champion team.
"As well as providing more fixtures for the teams already involved, this will also bring both a major cash injection into the competition and greater exposure across Europe.
"The implementation of the play-offs and the Magners League Grand Final this season has already seen a marked improvement in the competitiveness of the League and the arrival of two Italian teams will provide another boost to the competition.
"Operationally it will present new challenges, as well as opportunities, but by introducing Italy into the Celtic fold it will also dramatically broaden the appeal of our product.
"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."
FIR president Giancarlo Dondi added: "I am sure that the Magners League will bring benefits to all Italian rugby and will have a positive impact on the competitiveness of our national team."
The Magners League Italian move has been welcomed by the rugby hierarchy in Wales.
Regional Rugby Wales chief executive Stuart Gallacher said: "We have been supporting this move for some months.
"We are always looking to extra revenue and the Italians give us all two extra home games each and, given time, the Italians will be a force to be reckoned with in our league and it will make the league much more competitive.
"They have great plans to bring all their international players back from Europe to Italy and that would raise the profile of the tournament.
"With a population of 60 million and rugby starting to take hold, the potential of Italy is vast."
While Welsh Rugby Union chief executive Roger Lewis added: "For the last two years it has been our ambition to deliver an enhanced tournament which will expand and continue to develop as a showcase for some of the very best of northern hemisphere rugby.
"It will prove a great incentive for all four Welsh Regions to test themselves against teams from a nation which is emerging as a real strength in the European game."
The inclusion of Italian sides continues the Magners League revamp, which began this season with a new play-off and Grand Final system to decide the champions.
The Magners League began life as the Welsh-Scottish League in 1999 before growing into the Celtic League with the inclusion of Ireland in 2001.
The first moves to expand the tournament into new territories were made in 2005, with the proposal of a 24-team Rainbow Cup featuring teams from Wales, Scotland, Ireland, Italy and South Africa.
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