UPDATE 1-U.S. lawmakers agree to fund gov't through Dec. 18

(Adds detail, background)

WASHINGTON, Oct 27 (Reuters) - U.S. congressional negotiators on Tuesday agreed on a stopgap measure that would fund government operations through Dec. 18 while they finalize spending bills for the current fiscal year.

Without the agreement, which must still pass both houses of Congress, the U.S. government would be forced to shut down on Nov. 1.

The measure approved by House and Senate negotiators would fund government operations largely at their current levels.

It was attached to a bill that boosts spending for environmental protection for the current fiscal year, which started on Oct. 1.

The move took at least one lawmaker by surprise.

"The first time I saw this piece of paper was about three minutes ago," said Democratic Senator Dianne Feinstein, who heads the subcommittee that handles the environmental-spending bill.

Congress has already passed one stopgap measure to buy time while they finalize the 12 spending bills that fund government operations, but it expires on Oct. 31.

Last-minute budget wrangling has become an annual event in Congress, which last completed its spending bills on time in 1994.

The federal government was temporarily shut down in 1995 and 1996 due to a spending dispute between Democratic President Bill Clinton and Republicans who controlled Congress at the time.

(Reporting by Andy Sullivan; Editing by Eric Walsh)