UPDATE 1-Slack shares slump on full-year billings forecast, jump in expenses

(Adds analyst estimates, forecast, billings)

March 12 (Reuters) - Slack Technologies Inc forecast lower-than-expected billings for fiscal 2021 on Thursday and saw its expenses soar in the fourth quarter, sending its shares down more than 20% aftermarket.

The company said it expects billings of $970 million to $1 billion for fiscal 2021, lower than analysts' average estimate of $1.02 billion, according to IBES data from Refinitiv.

The office messaging platform's total operating expenses in the quarter leaped 67% to $248.7 million.

Slack said it had over 110,000 paid users at the end of the fourth quarter, up 25% from last year.

This helped the company post revenue of $181.9 million, a 49% rise from a year earlier, above analysts' average estimate of $174.1 million, according to IBES data from Refinitiv.

However, net loss attributable to common stockholders widened to $89.0 million in the fourth quarter ended Jan. 31, from $36.3 million a year earlier.

Excluding items, the company posted a loss of 4 cents per share. (Reporting by Bharath Manjesh in Bengaluru; Editing by Krishna Chandra Eluri)