Omnicom serves over 5,000 clients in 70 countries, including McDonald’s Corp and Apple. The memo did not mention clients by name and it is not clear if any have paused Twitter advertising spending.

The move, first reported by tech news site The Verge, emphasises a growing scepticism among agencies and brands about the micro-blogging site’s future since Elon Musk’s US$44 billion takeover.

The Tesla CEO has blamed civil rights groups lobbying Twitter advertisers to boycott the service until Musk clarified how he would control misinformation and hate speech on the service for a “massive” revenue drop.

“Twitter’s ability to maintain their previous level of brand safety measures and effectiveness seem impeded in the immediate term,” according to the memo.

“Whilst OMG believes this is unlikely to result in a significantly higher risk environment for advertisers, the risk of being associated with unsafe content could rise and as such should be considered when deciding on use of the platform.”

Ad sales represented over 90% of Twitter’s revenue in the second quarter.

Last month, US automaker General Motors said it had temporarily halted paid advertising on Twitter.