By Clare Jim and Saeed Azhar

HONG KONG/SINGAPORE (Reuters) - OCBC, Singapore's third-biggest bank, has agreed to buy ING's private banking unit in Asia for $1.5 billion, sources said on Thursday, a surprise outcome in a complex, drawn-out auction

OCBC competed against HSBC and fellow Singapore bank DBS for the asset.

OCBC's shares were suspended pending an announcement, which sources said was due at noon.

The Singapore bank is an unexpected winner in a deal bankers earlier had said could be clinched by HSBC. It was advised by Goldman Sachs , sources said.

DBS, which was also shortlisted in the final bidding process, dropped out of the race a few weeks ago, a source said.

The deal could be transformational for OCBC, whose private bank is smaller than similar units of Singapore's two other listed banks and in 2006 lost a third of its bankers to foreign rivals.

The sale of the Asian private bank network is the third major asset disposal for ING in less than a month and the second in the Asia Pacific region.

On Sept. 25, it sold its stake in a wealth management joint venture in Australia and New Zealand to partner ANZ for 1.1 billion euros (2.28 billion Singapore dollars).

On Oct. 7, it struck a deal to sell its Swiss private banking unit to Julius Baer for 344 million euros.

While the ANZ deal was a surprise, the sale to Baer had been rumoured for months.

ING is in the midst of raising 6 billion euros to 8 billion euros through asset sales under a restructuring program it announced in April. It plans to ultimately exit 10 of the 48 countries where it does business.

The restructuring follows 10 billion euros in state aid ING received in Oct. 2008 and a 22 billion euro asset guarantee it received from the Dutch state in Jan. 2009.

Both its restructuring plan and the asset guarantee are under review by the European Commission, though the asset guarantee in particular has received a frosty reception.

On Sept.15, the EU extended its review of the facility, saying it appeared the Dutch state may have paid too much for the assets and ING might have gotten an unfair deal.

That review is expected to be completed by the end of this year, European Competition Commissioner Neelie Kroes told the Dutch parliament on Oct. 13.

Analysts' worst-case scenarios envision a hit to ING of up to one billion euros if the portfolio is repriced - enough to potentially push its core Tier I capital ratio below 7 percent.

The asset sale program is seen as one potential way to offset a capital hit, though analysts have been unhappy with the sales thus far. In particular, they have found fault with both the prices paid and the fact that ING is selling assets in a still-depressed market.