A special meeting of the IRB's council, held on Friday morning, rejected proposals to reduce the number of participating sides to 16.
The length of the World Cup, which spans six weeks, and concerns that the group stages contain too many mismatches led to calls for a return to four pools of four.
But the unqualified success of France 2007, combined with the impressive performance of smaller nations such as Georgia and Tonga, convinced the IRB to resist making changes.
IRB and Rugby World Cup Limited chairman Dr Syd Millar said: "Prior to the 2007 World Cup, the IRB stated that it would review the number of participating teams for future tournaments.
"The reason for this review was to ensure that the tournament remains competitive and commercially attractive.
"The developing nations at this year's World Cup have produced significantly enhanced performances on four years ago.
"This is a direct result of the global Â£30million IRB strategic investment programme that was established in 2005.
"Based on this, plus the likelihood that surplus revenue from this year's World Cup will provide further funds for future investment in developing nations, the council had no hesitation in approving the recommendation from the RWCL Board that 20 teams participate in 2011."
New Zealand will stage the next World Cup and Millar insisted the decision to persevere with 20 teams received full backing from the hosts.
It had been suggested a lack of infrastructure in New Zealand would prompt them to push for the reduction.
But Millar said: "The successful New Zealand tender bid for 2011 was based on 20 teams.
"At the council meeting the motion maintaining the number of teams at 20 for 2011 was proposed by New Zealand council member Graham Mourie."
The playing window for the 2011 World Cup will be September to October 2011 with the final taking place on the weekend of October 22-23.
The number of automatic qualifiers has been increased from eight to 12.
As the top three teams in their 2007 pools, South Africa, England, Argentina, France, Australia, New Zealand, Scotland, Fiji, Wales, Ireland, Tonga and Italy have thus automatically qualified.
The qualification system for the remaining eight places is currently being reviewed with regional qualification starting in 2008.