Sept 4 (Reuters) - United Airlines on Thursday

rushed to cash in on rival Spirit Airlines' financial troubles,

beefing up its footprint in the bankrupt discount carrier's main

markets, including Fort Lauderdale, Orlando and Las Vegas.

Spirit Airlines, which filed for its second bankruptcy

protection last week, has been shrinking its operations and

retreating from a number of markets, which has opened up an

opportunity for rivals.

Last week, Frontier Group ( ULCC ) introduced flights to

Spirit's strongholds.

"If Spirit suddenly goes out of business, it will be

incredibly disruptive, so we're adding these flights to give

their customers other options if they want or need them," said

Patrick Quayle, United's senior vice president of Global Network

Planning and Alliances.

United will start selling tickets on Thursday for new

flights to 15 cities, where Spirit operates. The Chicago-based

airline said it will fly larger aircraft between Chicago and New

York LaGuardia to help customers outside of its hubs to connect

to the newly added flights.

"We expect these carriers to continue to see a sizable

benefit from Spirit's retrenchment, despite having less total

overlap," TD Cowen analyst Tom Fitzgerald wrote in a note

earlier this week.

(Reporting by Nathan Gomes in Bengaluru and Rajesh Kumar Singh

in Chicago; Editing by Shailesh Kuber, Sriraj Kalluvila and

Shinjini Ganguli)