An $11 billion merger of two of Australia's biggest gaming companies, Tabcorp and Tatts, is facing a legal challenge from the national competition watchdog.

The massive merger proposal last month won crucial regulatory approval in the Australian Competition Tribunal, in a decision that angered rival betting companies and dismissed many concerns surrounding the merged entity's inflated market power.

Tabcorp and Tatts took the unusual step of applying directly to the tribunal and bypassing the usual approval process of the Australian Competition and Consumer Commission (ACCC), which had voiced concerns about diminished competition for licences and broadcast rights.

On Monday, ACCC chairman Rod Sims announced the commission had applied to the Federal Court for a "judicial review" of the decision.

"The ACCC is alleging the tribunal made three reviewable errors," Mr Sims said. 

"It is therefore seeking clarification of these three points of law, which are central to the tribunal's assessment of Tabcorp's proposed acquisition of Tatts."

The legal challenge risks dashing Tabcorp's hopes of completing the merger deal within weeks.

More to come