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The board of net overseer Icann gave initial approval for the creation of the .xxx domain at its conference in Brussels.
Icann's approval will kick off a fast-track process to get the porn-only domain set up.
ICM Registry, which is backing the domain, said .xxx would make it easier to filter out inappropriate content.
The decision ends a long campaign by ICM Registry to win approval.
Stuart Lawley, chairman of ICM, welcomed the decision and said it was "great news for those that wish to consume, or avoid, adult content".
Icann, the Internet Corporation for Assigned Names and Numbers, said the next step would be talks with ICM Registry to ensure that it still had sufficient backing from the adult industry to justify its creation.
Icann oversees the net's addressing system and demands that new top level domains, such as .biz, must be "sponsored" in that they must serve the needs of a defined community.
ICM has said it expected the first .xxx domains to go live in early 2011. It said it had more than 110,000 pre-reservations for .xxx domains.
During the debate on approval, Icann board members said mistakes had been made in denying approval before now.
The denial was "not consistent with the application of neutral, objective and fair documented policy".
Some board members said they were personally "uncomfortable" with the decision to approve .xxx but could not block it because ICM had followed Icann's rules for setting up a new top-level domain.
Initial approval for .xxx was given in 2005 but this was rescinded following protests from politically conservative groups. Icann also said it was worried that it would be asked to police content - a role it was not created for.
In February 2010, a panel of US judges said Icann erred in denying the approval of the domain. Icann accepted that ruling and began a public consulation process on whether the domain should be approved.