The world's fifth biggest smartphone maker said yesterday that it expected revenues for the final three months of 2011 to be lower compared to last year.
The company has previously predicted growth of 20% to 30% in the fourth quarter, but it is now expecting flat growth compared to the same period last year, when revenues were T$104bn ($3.42bn).
This would mean a 23% fall compared to the third quarter, when HTC posted record quarterly revenue, but the company said that it expected revenue growth to pick up in the first half of 2012.
The downbeat announcement sent shares in HTC tumbling 6.9% to 560 New Taiwan dollars on Thursday, the maximum drop allowed in one day, and reaching their lowest level in 16 months.
Analysts reacted with the shock to the announcement, filed with the Taiwan Stock Exchange.
In a note to clients reportefd by the BBC, Sanford Bernstein from Pierre Ferragu said: "This new guidance takes us by complete surprise and is at odds with recent discussions we have had with distribution channels, especially in Europe."
HTC had warned in October that its fourth quarter revenue was showing signs of slowing, predicting T$125bn to 135bn (£2.6bn-£2.9bn), compared with T$135.8bn in the previous quarter.
The company did not give a specific reason behind the further downward forecast, but said that it expected sales to be reduced by about 20% as operating conditions remain tough.
Analysts believe that the difficult fourth quarter could be down to a paucity of new products from HTC and the increased competition from Apple, Samsung and others.
The company has also been locked in a series of patent battles with its rivals, including the loss of a graphics technology battle with Apple this week after a US regulator ruled against the firm.
Also this week, it was claimed that Facebook has selected HTC to produce the first ever smartphone branded and fully integrated with the world's biggest social network.