WASHINGTON (Reuters) - T-Mobile US (>> T-Mobile Us Inc) plans to overtake Sprint Corp (>> Sprint Corp) as the No. 3 wireless U.S. carrier in subscribers by the end of the year, Chief Executive Officer John Legere, tweeted on Thursday.

The declaration came a day after T-Mobile announced it surpassed Sprint as the No. 1 wireless provider for prepaid customers, with 15.64 million customers, compared with Sprint's 15.19 million.

The figures put T-Mobile, which calls itself the "uncarrier," well ahead of rivals AT&T (>> AT&T Inc.) and Verizon (>> Verizon Communications Inc.), in the prepaid market. Verizon and AT&T hold a larger share of the country's more profitable postpaid subscriber base.

"I predict the #uncarrier will overtake @Sprint in total customers by the end of the year! There, I said it!," tweeted Legere.

While T-Mobile has been adding record numbers of subscribers through promotions and campaigns, Sprint customers have been leaving in droves due to service problems arising from carrier's network overhaul.

Earlier this week, Sprint's parent company SoftBank Corp (>> Softbank Corp) walked away from an offer to buy T-Mobile and named Miami businessman Marcelo Claure its new chief executive.

Legere, known for his outspoken style, did not waste time in engaging with his new competitor, and tweeted at Claure on Thursday "Sorry @marceloclaure, you're already behind!."

While T-Mobile's aggressive strategy has helped the company turn around, its parent company, Deutsche Telekom (>> Softbank Corp) has been concerned that its lack of low-frequency spectrum and fixed-line infrastructure is still hampering its ability to compete.

The company will have to spend an estimated $5 billion to $10 billion in the upcoming auction of radio frequency waves, according to one analyst, the first public auction for valuable low-frequency airwaves in nearly a decade

T-Mobile shares were down 3.21 percent and Sprint shares were up 1.5 percent after falling nearly 20 percent on Wednesday.

(Reporting by Marina Lopes; Editing by David Gregorio)

By Marina Lopes