* No easy fix to regulatory concern
* Deal will be delayed until 2016
* Brookfield working with regulator to address concerns
* Shares down on delay concerns (Adds comments from Brookfield)
By Byron Kaye
SYDNEY, Oct 15 (Reuters) - An Australian regulator raised antitrust concerns over a planned $6.5 billion takeover by Canada's Brookfield Asset Management BAMa.TO of freight firm Asciano Ltd AIO.AX , potentially jeopardising the country's biggest inbound deal in four years.
The proposed deal, which would be the largest-ever purchase of an Australian firm by a Canadian company, would give Brookfield Asciano's rail network and train operations in two of the country's eight states, the Australian Competition and Consumer Commission (ACCC) said in a statement on Thursday.
"The ACCC is concerned that the vertical integration will lead to a substantial lessening of competition in related markets for the supply of above rail haulage services in (Western Australia) and Queensland," the commission's chairman Rod Sims said.
While the ACCC made no mention of blocking the deal in its entirety, the statement raised uncertainty about whether Brookfield can overcome the regulator's concerns without carving out large parts of the target company's business - a move which would make it less appealing to buy.
Asciano shares fell 8 percent to A$7.89 by mid-afternoon. The shares are now below both Brookfield's A$9.15 offer price and below their level before the companies first said in August that they had agreed to the deal. The broader market .AXJO was trading higher.
"It's really difficult, quite frankly, to see an easy solve to this," said Georgina Foster, a competition lawyer at Baker & McKenzie in Sydney.
In cases where a company wants to acquire several parts of an industry, the ACCC is typically hesitant to approve the deal, concerned that other players would be shut out, Foster said.
Sims said the ACCC already regulates rail freight access, but noted that "where the owner of such infrastructure vertically integrates with one of a very limited number of users of the infrastructure ... an access regime may not be capable of averting a substantial lessening of competition".
Adding to the imposition, the ACCC said it will now give a final ruling by Dec. 17, scuppering the companies' assurances to investors that they would have the sale completed by the end of the year.
In a statement, Asciano said Brookfield "is working closely with the ACCC to address its concerns", and that it expects the takeover to proceed in January, rather than December as planned.
Brookfield said it was confident it could satisfy the ACCC's concerns "through a combination of the existing regulatory frameworks and commitments given by Brookfield, which would be consistent with previous ACCC clearances in similar circumstances".
($1 = 1.3691 Australian dollars)