"TikTok workers have long been sounding the alarm over the real-world costs of cutting human moderation teams in favour of hastily developed, immature AI alternatives," CWU National Officer for Tech John Chadfield said.
He added the cuts had been announced "just as the company's workers are about to vote on having their union recognised".
But TikTok said it would "maximize effectiveness and speed as we evolve this critical function for the company with the benefit of technological advancements".
Impacted staff work in its Trust and Safety team in London, as well as hundreds more workers in the same department in parts of Asia.
TikTok uses a combination of automated systems and human moderators. According to the firm, 85% of posts which break the rules are removed by its automated systems, including AI.
According to the firm, this investment is helping to reduce how often human reviewers are exposed to distressing footage.
Affected staff will be able to apply to other internal roles and will be given priority if they meet the job's minimum requirements.