Work messaging platform Slack has filed an antitrust claim against Microsoft, claiming the tech giant’s rival app Teams has an unfair advantage.

Slack said Microsoft’s bundling of Teams within Office 365 software was “illegal and anti-competitive practice” and that the tech giant was “abusing its market dominance”.

The complaint will now be reviewed by the European Commission.

The BBC has contacted Microsoft for comment.

Slack argued that it was seeking a “level playing field” and suggested that, by offering Teams to Office 365 users, Microsoft was making it harder for Slack to sell its own software to the market.

“We want to be the 2% of your software budget that makes the other 98% more valuable; they want 100% of your budget every time,” said Jonathan Prince, Slack’s vice president of communications and policy.

With millions more employees working from home during the pandemic, rivalry over the technology that makes remote working possible has deepened.

Microsoft Teams users grew from 44 million in March to 75 million in April and Slack has also seen a large rise in users in recent months, reaching 12.5 million by late March.

The antitrust claim contrasts with comments made by Slack’s chief executive Stewart Butterfield in May, when he told CNBC: “What we’ve seen over the past couple of months is that Teams is not a competitor to Slack.”

He went on to mention the fact that Microsoft bundles Teams with Office 365 and argued the growth of Teams users during the past three years was unimpressive.

“They still only have 29% which means 71% of [Office 365] users have said ‘No thank you’.”