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WASHINGTON, Dec 16 (Reuters) - The U.S. intelligence community has committed to providing as soon as next month a public estimate of the number of U.S. persons whose electronic communications are ensnared under a surveillance authority intended for foreign espionage, according to a bipartisan group of congressional lawmakers’ letter that Reuters saw.

The decision would reverse the government’s longstanding position that calculating such a number may be technically impossible and would require privacy intrusions exceeding those raised by the actual surveillance programs.

It also comes as Congress is expected to begin debate in the coming months over whether to reauthorize or reform the surveillance authority, known as Section 702, a provision that was added to the Foreign Intelligence Surveillance Act in 2008.

The letter, sent on Friday to National Intelligence Director James Clapper, said his office and National Security Agency officials had briefed congressional staff about how the intelligence community intends to comply with the lawmakers’ disclosure request.

Clapper’s office did not immediately respond to a request for comment.

The 11 lawmakers, all members of the U.S. House Judiciary Committee, termed their letter an effort to “memorialize our understanding” of the intelligence community’s plan to provide an estimate in real numbers, not percentages, as soon as January that can be shared with the public.

“The timely production of this information is incredibly important to informed debate on Section 702 in the next Congress- and, without it, even those of us inclined to support reauthorization would have reason for concern,” the letter said.

Section 702 will expire on December 31, 2017, absent congressional action. It enables two internet surveillance programs called Prism and Upstream that were revealed in a series of leaks by former NSA contractor Edward Snowden more than three years ago.

Intelligence officials have said that data about Americans is “incidentally” collected under Section 702, due to a range of technical and practical reasons. Critics have assailed such collection as back-door surveillance of Americans without a warrant.

Clapper, who is stepping down next month, suggested in April that providing an estimate of Americans surveilled under Section 702, a figure some have said could tally in the millions, might be possible, while defending the law as “a prolific producer of critical intelligence.”

Republicans James Sensenbrenner, Darrell Issa, Ted Poe and Jason Chaffetz signed the letter, in addition to Democrats John Conyers, Jerrold Nadler, Zoe Lofgren, Hank Johnson, Ted Deutch, Suzan DelBene and David Cicilline. (Reporting by Dustin Volz; Editing by Lisa Von Ahn)