Slack set a guide price of $26 a share, but rose 60% at the start of trading before easing back to finish at $39.

The company chose a direct listing on the stock market, rejecting the use of traditional advisers and underwriters who manage the price of new stocks.

That opened the possibility of wild swings in the price as traders try to assess where the shares might settle.

The jump in the share price put the value of the company at $25bn.

Slack is the second big tech firm to go the direct route, after music streaming service Spotify used the method last year.

"We think the jury is out on whether this is the right move or not," Kathleen Smith, a listing expert at Renaissance Capital, said ahead of the start of trading.

"Looking at Spotify, it takes a little time for the stock to get established after a direct listing."

Slack's listing fees are expected to be about $22m. When Snap went public in 2017, it paid about $85m to its financial advisers.

Spotify's listing is generally regarded as a success, although the shares now trade about 15% below their debut price.

If Slack can also make a success of its direct listing, it could have implications for how future tech firms come to market, including for Airbnb.

Slack's software replaces emails by grouping messages around subjects, projects and teams. It means that flooding people with irrelevant emails can be cut.

The software has become increasingly popular, with HSBC and Ford among some of the big corporate users. It has about 100,000 paying customers.

Founder Steward Butterfield, who developed the photo app Flickr, says Slack is a revolution in corporate communication.

But like many big tech firms coming to market, Slack has never made a profit. Although revenue rose 80% to $400m in 2018, losses were $144m.

And some analysts are worried that Slack is competing in an increasingly crowded market. Microsoft offers Teams, a free chat app add-on for its Office365 users.

Slack's debut follows a spate of much-anticipated technology listings, some of which, including Uber Technologies and Lyft, had disappointing starts to trading.