(Reuters) – An expert group reviewing the cause of a $1.4 billion cost overrun at a Mongolian mine run by Rio Tinto said it was caused by the miner's mismanagement, the Wall Street Journal reported on Monday, citing a report.

Costs to expand the Oyu Tolgoi mine, Rio's biggest copper growth project, have ballooned up to $6.75 billion from Rio's original budget of $5.3 billion in 2016, and this has led to a friction over funding with Turquoise Hill.

Turquoise Hill, in which Rio has a 50.8% stake, owns 66% of Oyu Tolgoi, one of the world's largest-known copper and gold deposits. The rest is held by the Mongolian government.

The report, which was commissioned by the owners of the copper project, said the cost overrun was not due to unfavorable rock conditions as blamed by the one of world's largest miner, the WSJ reported.

"This confidential report will be considered by the OT (Oyu Tolgoi) Board and Rio Tinto will engage with the OT Board as soon as we have had the opportunity to review the report in detail," Rio Tinto said in an emailed statement.

There was no evidence that the quality of the rock and general ground conditions were significantly different to that forecast by the miner's owners in 2016, according to the report.

The WSJ report said the U.S. Securities and Exchange Commission and British regulators were looking into the matter.

Turquoise Hill Resources did not immediately respond to a Reuters request for comment.

(Reporting by Priyanshi Mandhan in Bengaluru; Editing by Subhranshu Sahu)