* Third attempt by companies at $4.9 bln takeover
* Competition tribunal has approved the deal
* Federal Court upholds appeal from antitrust regulator (Adds comment from Tabcorp, Tatts and the ACCC, adds shares)
By Byron Kaye and Tom Westbrook
SYDNEY, Sept 20 (Reuters) - An Australian court upheld on Wednesday an appeal against betting firm Tabcorp Holdings Ltd's agreed A$6.15 billion ($4.9 billion) buyout of lotteries operator Tatts Group Ltd.
Tabcorp and Tatts had billed their third attempt to join since 2006 as a way to create a domestic gambling powerhouse to fend off online rivals like Britain's William Hill and Ireland's Paddy Power.
The deal was cleared in June by the Australian Competition Tribunal (ACT), a court-affiliated body, but antitrust regulator the Australian Competition and Consumer Commission (ACCC) appealed the decision.
"The court orders that the decision of the tribunal ... be set aside (and) be referred back to the tribunal for further consideration," three Federal Court judges wrote, adding that they would publish their reasons in five days.
The decision marks a potentially time-consuming setback for a deal announced a year earlier which has already been delayed by the ACCC's court challenge.
Representatives of Tabcorp and Tatts were not immediately available for comment.
Tabcorp and Tatts took the unusual step of applying to the ACT after the usual arbiter, the ACCC, had raised concerns about the deal.
The ACCC then sought a review on the grounds that it believed the ACT had misused certain tests to determine if the deal would hurt competition, and that it had given inappropriate weightings to data about the effects of the takeover.
Shares in Tabcorp and Tatts were in a trading halt on Wednesday.
$1 = 1.2473 Australian dollars Reporting by Byron Kaye and Tom Westbrook; Editing by Stephen Coates