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By Ellis Mnyandu and Jonathan Spicer

NEW YORK, July 2 (Reuters) - The New York Stock Exchange suffered its third technical glitch in less than a month on Thursday, this one compelling the centuries-old floor to extend the trading day by 15 minutes to finish executing orders.

Brokers on the Wall Street trading floor, run by NYSE Euronext (NYX.N: Quote, Profile, Research)(NYX.PA: Quote, Profile, Research), had trouble routing orders for more than an hour early in the session on Thursday. A spokesman said it was an isolated issue that did not impact the broader electronic trading system, and that regular order processing resumed by 10:50 a.m. (1450 GMT)

The Big Board briefly halted trading in some stocks, including Regis Corp (RGS.N: Quote, Profile, Research) and Avon Products (AVP.N: Quote, Profile, Research). Trading was extended to 4:15 p.m. (2015 GMT) from the regular 4 p.m. close, and the exchange was reviewing if trades would need to be canceled.

The problem follows two others at the NYSE -- a software glitch June 12, and an order-matching problem June 26 -- as it scrambles to retain market share and upgrade systems in order to keep pace with smaller, faster rivals such as BATS Exchange and Direct Edge.

On Wednesday, the Big Board launched a new order processor it said reduces order execution time from 105 milliseconds to 5 milliseconds. The explosion in automated trading has led many high-frequency traders to measure trading in even shorter microseconds.

NYSE Euronext and cross-town competitor Nasdaq OMX (NDAQ.O: Quote, Profile, Research), which runs the Nasdaq Stock Market, are shedding market share to the smaller venues and non-displayed markets, and face the wider possibility that a protracted recession and regulatory changes could dampen trading volumes.

Nasdaq suffered a smattering of technical issues over the past year, including the cancellation of some late trades in Google Inc (GOOG.O: Quote, Profile, Research) in September. (Reporting by Ellis Mnyandu and Jonathan Spicer; Editing by Jan Paschal, Gary Hill)