* Competition regulator rejects Brookfield's assurances

* Decision a blow to Australia's biggest Canadian takeover

* Bodes well for rival proposal from smaller Qube Holdings (Adds Brookfield comment, updates shares)

By Byron Kaye

SYDNEY, Nov 26 (Reuters) - An Australian regulator on Thursday knocked back an attempt by Canadian infrastructure giant Brookfield Asset Management Inc to overcome competition concerns holding up its A$9 billion ($6.53 billion) bid for port and rail firm Asciano Ltd.

The decision is a blow to a takeover proposal backed by Asciano and boosts the prospects of a rival bid from smaller Australian logistics firm Qube Holdings Ltd.

The regulator said it would make a final ruling on Australia's biggest inbound takeover in four years - and its biggest buyout by a Canadian firm - on Dec. 17.

The Australian Competition and Consumer Commission dismissed Brookfield's "long-term behavioural undertakings" to address worries that the deal would give it both the rail tracks and freight trains in some locations, potentially enabling it to cut out other freight operators.

"After detailed consideration, the ACCC has concluded that the undertakings are not acceptable," ACCC Chairman Rod Sims said in a statement, without specifying the undertakings.

The ACCC said it made its decision partly on the basis of industry feedback which had raised doubts that Brookfield's promises would be enforceable or effective.

Since Asciano agreed to Brookfield's offer in August, the ACCC has raised what it called "red light" concerns that the Canadian firm would control too much of the freight supply chain.

The regulator's rejection of Brookfield's assurances is a win for Qube, which in October made a slightly higher indicative offer for Asciano.

Asciano shares were steady and Qube shares were up as much as 4 percent in a higher overall market.

In a statement on Thursday, Brookfield said it is "evaluating the variety of available alternatives for addressing those issues that the ACCC has determined cannot be resolved through undertakings relating to conduct alone". These included new unspecified "structural undertakings".

Earlier this week Qube Chairman Chris Corrigan, a former head of Asciano's port unit, revealed that Qube had been in talks with Asciano about the possibility of buying its port business when Brookfield made an offer for the entire company.

($1 = 1.3793 Australian dollars) (Reporting by Byron Kaye; Editing by Stephen Coates and Simon Cameron-Moore)