The Google-backed communication app developer has raised an initial $135m from investors that could rise to $160m before the round is closed.

Slack, the US-based creator of a workplace communication app backed by investors including internet company Google, has agreed up to $160m in new funding, the Wall Street Journal reported yesterday.

The company has secured $135m from investment firm Horizons Ventures, which provided $50m, Institutional Venture Partners, Index Ventures and DST Global, and has the option to raise a further $25m from these new investors or its existing backers, according to people familiar with the matter.

Slack has developed a freemium…