South Korea's Hyundai Motor Group has picked Goldman Sachs to advise it on a bid for a $2.1 billion stake in builder Hyundai Engineering & Construction, a source close to the matter said on Wednesday.

Hyundai Motor Group, which also includes Kia Motors, Hyundai Steel, HMC Investment Securities  and unlisted builder Hyundai Amco, has yet to decide which firm will lead the bidding or whether they'll form a consortium, the source said.

The source declined to be named as the country's largest auto group has yet to make an official announcement on the bidding.

Hyundai Motor declined to comment and said nothing has been decided.

Korea Exchange Bank (KEB), Woori Bank, Korea Finance Corp and other financial firms aim to select a preferred bidder for the sale of their 35 percent stake in Hyundai Engineering worth 2.5 trillion won ($2.1 billion) at current market value before the end of this year.

A KEB official said that sales advisers will post a public notice of the sale on September 24.

Hyundai Motor Group, if it participates in the bidding, will be pitted against trading-to-tours conglomerate Hyundai Group to snap up the former flagship company of Hyundai Group, which was split into auto, heavy industry and trading groups after the Asian financial crisis in the late 1990s.

The auto group, which is ranked the world's 5th biggest car maker, emerged cash-rich after the split and is better positioned financially for a takeover than the more leveraged Hyundai Group.

But concerns of little synergy benefit from the move have capped the automaker's share prices.

Hyundai Group said in August that it would bid for Hyundai E&C. Hyundai Group unit companies include Hyundai Merchant & Marine Corp, Hyundai Elevator, Hyundai Securities and North Korean tour operator Hyundai Asan.

($1=1177.0 Won)

(Reporting by Hyunjoo Jin; Editing by Jonathan Hopfner and Muralikumar Anantharaman)