Virgin Media has confirmed its call centre in Swansea will close by July 2019, with the loss of almost 800 jobs.
The Welsh Government said the decision to close Matrix Court was "very disappointing".
Some workers are facing redundancy and others have been offered the prospect of relocating to Virgin Media's other sites across the UK.
A total of 772 jobs will go in Llansamlet, 552 staff positions and 220 sub contractors.
Swansea council leader Rob Stewart said the city was working with other employers including Tui and Admiral to explore local redeployment opportunities.
Virgin Media plans to cut its number of UK customer service bases from eight to four, centralising operations in Manchester.
Chief executive Tom Mockridge said it had considered counter-proposals from employees but decided to proceed with its original plans.
"We are now working with and supporting our people who have been asked to relocate or transfer, who are in a role at risk of redundancy or who may wish to take another role with Virgin Media or one of our partners," he said.
The government said its priority was to support affected staff.