UPDATE 1-U.S. Fed balance sheet grows to another record size

(Adds details)

NEW YORK, July 7 (Reuters) - The U.S. Federal Reserve's balance sheet expanded to a record size in the week ended July 6 as the central bank bought more bonds in an effort to support the economy, Fed data released on Thursday showed.

The purchases were part of its $600 billion program, dubbed QE2, aimed at stimulating investment and economic activity. The final purchase under the $600 billion program was done on June 30, although the central bank continues to buy Treasuries under a previously announced program using the proceeds from maturing agency bonds and mortgage-backed securities.

The balance sheet -- a broad gauge of Fed lending to the financial system -- rose to $2.854 trillion in the week ended July 6 from $2.849 trillion in the week ended June 29.

The Federal Reserve's holdings of Treasuries totaled $1.625 trillion as of July 6, versus $1.617 trillion as of June 29. Primary borrowing at the Fed's discount window averaged $12 million a day in the week ended July 6, versus $18 million a day in the week ended June 29.

Fed holdings of agency debt totaled $115.1 billion as of July 6, versus $116.7 billion as of June 29. Fed holdings of agency mortgage-backed securities were unchanged on the week at $908.85 billion. (Reporting by Chris Reese: Editing by Chizu Nomiyama)