UPDATE 1-US antitrust enforcers to revamp merger guidelines

* FTC chief says 'the time has come'

* Obama administration expected to be tougher on mergers (Adds Varney comment, M&A background)

By Diane Bartz

WASHINGTON, Sept 22 (Reuters) - U.S. antitrust enforcers plan to revamp merger guidelines, amid expectations that the Obama administration will give corporate combinations a tougher review.

"The 1992 guidelines explicitly stated that they would be revised from time to time," Federal Trade Commission Chairman Jon Leibowitz said on Tuesday at a conference at the Georgetown University Law Center. "We think the time has come to do that."

The FTC and Justice Department share the job of ensuring that mergers do not violate antitrust law.

The corporate community has been bracing for the Obama administration to be tougher than its Republican predecessor in assessing mergers and the actions of companies that dominate their markets.

One sign of a tougher process could the Justice Department's opposition to Google Inc's (GOOG.O) controversial deal with authors and publishers that would allow the search engine giant to create a massive online digital library.

In his prepared remarks, Leibowitz said that as they currently stand, the merger guidelines "clearly exaggerate the extent to which the agencies follow a single, rigid, step-by-step approach in merger analysis.

"We are announcing the (guidelines) project now, but do not have a draft, let alone anything to release at this point," he said. "On the contrary, we are soliciting comments."

In her remarks to the conference, Christine Varney, assistant attorney general in charge of the department's Antitrust Division, seemed interested in the re-adoption of the Herfindahl-Hirschman Index, or HHI, a way to calculate a market's concentration.

HHI has not been used in years, she said.

David Balto, a former FTC policy director, said the re-adoption of HHI could mean regulators would challenge a lot more mergers.

The move to revamp the guidelines, which were adopted in 1968 and revised in 1982 and 1992, comes as tentative signs emerge that the mergers and acquisitions market is recovering.

Kraft Foods Inc's (KFT.N) unsolicited offer for Cadbury Plc CBRY.L and Walt Disney Co's (DIS.N) deal to buy Marvel Entertainment Inc MVL.N indicate more interest in the M&A market, although deal volume for the year is likely to be down sharply from 2008.

In the three months from June to August, global M&A rose 29 percent compared to the preceding three months. In the second quarter, M&A was up 15.2 percent compared with the first quarter, according to data from Thomson Reuters. (Reporting by Diane Bartz; Editing by Lisa Von Ahn and John Wallace)