Unite has called for "urgent" talks about the future of PG Tips' factory in Manchester after the brand was sold by Unilever to CVC Partners for £3.8bn alongside Lipton.
The tea operation, known as Ekaterra, runs 34 brands and generated £1.7bn of revenues in 2020, Unilever said.
Unite has said it has been negotiating for three years over terms and conditions at the PG Tips factory at Trafford Park where the union has 300 production and engineering members.
The deal comes almost two years after Unilever started the process of reviewing and spinning off the operation, which is the world's largest tea manufacturer.
The acquisition is subject to regulatory approvals and is expected to complete in the second half of 2022.
The deal will not include Unilever's tea business in India, Nepal and Indonesia, the consumer group added.
Unite national officer Rhys McCarthy said: "The UK is a nation of tea drinkers and the loyal consumers, who put PG Tips in their shopping basket every week, will expect that UK workers are treated with decency and respect, not trampled over in the stampede for boardroom riches.
"Unite is determined to ensure that the security and future of the workers are at the centre of this sale.
"We are seriously concerned that CVC is a company with no track record in this sector and comes with a history of bringing instability and dis-investment to our high street.
"Unite has repeatedly urged Unilever not to succumb to the cult of quick growth that has wrecked so many sustainable businesses and the jobs that come with them simply to appease stock market players."
However Unilever's chief executive officer Alan Jope hailed the move as "further progress" as it continues to reshape its consumer portfolio.
He said: "The evolution of our portfolio into higher growth spaces is an important part of our growth strategy for Unilever.
"Our decision to sell Ekaterra demonstrates further progress in delivering against our plans.
"We look forward to seeing Ekaterra, with its strong brands and global footprint, prosper under CVC's ownership."
Pev Hooper, managing partner at CVC Capital Partners, added: "Ekaterra is a great business, built on strong foundations of leading brands and a purpose-driven approach to its products, people and communities.
"Ekaterra is well positioned in an attractive market to accelerate its future growth, and to lead the category's sustainable development.
"We look forward to working with the team to realise Ekaterra's full potential."
John Davison, chief executive officer of Ekaterra, said: "Ekaterra is a strong business with positive momentum and has an exciting future ahead under the new ownership of CVC.
"We look forward to the next stage of our journey as the world's leading tea business."
