BlackRock Inc. (BLK) is poised to buy Barclays PLC's (BCS) asset-management unit for between $12 billion and $14 billion in a deal expected to be announced Wednesday morning, according to a person briefed on the situation.

The purchase will transform New York-based BlackRock into the world's largest asset-management company, overseeing nearly $3 trillion. U.K.-based Barclays, which has been struggling to raise capital amid the credit crisis, will retain a 20% stake in the business, which is known as Barclays Global Investors. San Francisco-based BGI includes the fast-growing iShares ETF unit and a larger but less-prominent institutional money management operation.

BlackRock will use a combination of cash and stock to pay for Barclays Global Investors. The proposed deal trumps an earlier $4.4 billion agreement for Barclays to sell just the iShares business to private equity firm CVC Capital Partners. CVC will have five days to come up with another, higher bid. If it doesn't, Barclays will owe it a roughly $180 million breakup fee.

On Tuesday, shares of BlackRock rose $12.78, or 7.5%, to $182.14 as rumors of the deal circulated.

-Ian Salisbury; Dow Jones Newswires; 201-938-5219; ian.salisbury@dowjones.com