Slack Technologies, which runs a messaging service for businesses, is seeking an investment that would value the company at $3.5 billion to $4 billion, sources have said.

Slack is looking to raise $150 million to $300 million as part of the round, but a lead investor has not yet been selected, the sources said.

If successful, Slack’s fundraising round would stand out. Venture investment in start-ups dropped about 30 per cent in the fourth quarter compared with the previous one, according to research firm CB Insights.

Also during the last three months of the year, 26 per cent of mature start-ups raised money at lower valuations than in earlier rounds, according to a study by US law firm Fenwick and West. Even as start-up fundraising shows signs of slowing, a few companies are pushing ahead.

Slack, which has a base in Dublin, said it raised $160 million less than a year ago, valuing the company at $2.8 billion. It has raised more than $300 million in total and has also used its fundraising clout to create an $80 million venture fund with several of its investors to back Slack-related apps. Slack declined to comment on the current fundraising talks.

The Wall Street Journal reported Slack was seeking more than $150 million in an "up" round. Slack, which launched its current business about two years ago, said it now has 2.3 million daily active users. – (Bloomberg)