UPDATE 1-Mexico cuts interest rates for first time since 2009

* Banco de Mexico cuts main rate by 50 basis points

* Drags borrowing costs to record low of 4.0 percent

* One-off cut recognizes success in fighting inflation

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MEXICO CITY, March 8 (Reuters) - Mexico's central bank cut interest rates for the first time in nearly four years on Friday, taking borrowing costs to a new record low as policymakers bet they are winning the battle against inflation in Latin America's second-largest economy.

The Banco de Mexico cut benchmark borrowing costs by 50 basis points to 4.50 percent in a move predicted by only five of 21 analysts polled by Reuters last week.

Although policymakers had said at their last meeting that looser policy was possible if both lower growth and lower inflation continued, most had not expected the central bank to act so quickly.

The move surprised markets and most analysts who have watched the central bank hold its benchmark rate steady since mid-2009 and back away from previous signals that it might tweak borrowing costs in either direction.

"This change recognizes the success in the medium term in bringing down inflation and will help the economy adjust to a scenario of lower economic growth and inflation," the central bank said in a statement, noting that the cut was not the start of a cycle.

The statement made it clear the cut is a one-off aimed at bringing Mexican rates into line with easy money policies in major economies and reflects the central bank's view that inflation is on a steady downtrend towards its 3.0 percent target.

Policymakers shrugged off an acceleration in inflation to 3.55 percent in February and said slow growth and continued slack in the labor market would help keep prices in check.

Inflation is expected to tick up further in the next two months, giving the central bank a narrow window to act.

LONG TIME COMING

Mexico's growth is seen slowing this year to 3.5 percent, from a 3.9 percent expansion in 2012, and a sharp slump in exports and retail sales at the start of the year had fanned concerns about the impact on Mexico of weaker U.S. growth.

The cut, the first decision under a new composition of the Banco de Mexico board, is the culmination of a case governor Agustin Carstens has been making since his appointment in 2010.

Then, he argued that rock bottom rates in major economies had created a low-inflation atmosphere where Mexico too could cut credit costs without stoking prices. In early 2012, the central bank also signaled it would like to lower rates.

But both times the cut flirtation was stifled by global market turmoil hammering the peso and threatening to fan inflation through higher import prices.

Carstens may have been aided in his arguments this time by new board member Javier Guzman, who replaced Jose Sidaoui, seen as one of the board members most concerned about inflation.

The Banco de Mexico did not say whether the decision was unanimous -- information that will come in the minutes in two weeks -- but deputy governor Manuel Sanchez had urged caution on declaring inflation under control.

(Reporting by Krista Hughes and Michael O'Boyle; editing by Clive McKeef)