Staff working in NTL's Broadcast division are voting on recognition of broadcasting union Bectu.
The outcome of the ballot will decide whether Bectu continues to provide collective representation for staff at the company, renamed Arqiva earlier this year after being bought by Australian bank Macquarie.
After the sale was completed in February 2005 the firm - which operates transmitter sites for broadcasters and other users - gave notice that it planned to terminate the union's recognition agreement.
Talks at conciliation service Acas followed which resulted in an agreement between management and union to give staff a say in the future of the recognition agreement.
"This is a crucial decision for NTL staff and we are doing all we can to encourage maximum participation in the vote," said Bectu supervisory official Sharon Elliot.
"Union recognition is a right which is hard-fought for and we hope that staff... will recognise the importance of having a recognition agreement in place whether individuals choose at this time to be Bectu members or not," she said.
The ballot, which is being conducted by Electoral Reform Services runs until 22 August.