Smartphone maker HTC Corp said its sales fell nearly 68 percent in June, its biggest slump in more than two years, as the struggling firm continues to lose customers amid intensifying competition with nimbler manufacturing rivals.

Once a favourite with consumers and investors, HTC's popularity has dwindled with the emergence of rivals such as Apple, Samsung Electronics Co Ltd, and more recently China's Xiaomi Corp.

HTC said its June sales fell to T$2.2 billion ($72 million), from T$6.9 billion a year ago and T$2.45 billion in May.

"In the high-end segment, the sales of their flagship phone this year has been lower than expected, leading to lower market share," said an analyst at market research firm Trendforce who declined to be named.

"As for HTC's middle-end and entry-level series, the new models feature neither new specs nor high performance-price ratio, influencing the sales," the analyst said.

Trendforce expects HTC to launch new models less frequently this year, estimating production volume for 2018 at less than 2 million units.

Last week, HTC said it plans to slash nearly a quarter of its global workforce - or 1,500 jobs - through layoffs at its factory in Taiwan, underscoring troubles at the company that once sold one in 10 smartphones globally.

HTC stock has dropped nearly 30 percent so far this year.