The call centre employs 99 staff, of which 42 are nurse advisors and 39 are health advisors, with the remainder being administrative and management staff.
Unions met today with senior management and a consultation document is to be distributed tomorrow (Tuesday).
NHS Direct has promised to minimise redundancies and will guarantee similar posts are made available at an alternative site in England.
The nearest call centres are in Beckenham, South London and Ilford in Essex.
Ealing PCT, which owns the building the call centre operates from, has decided it required the office space occupied by NHS Direct.
NHS Direct interim chief executive Paula Higson said: ‘Ealing PCT’s decision means that we have no option but to move out of Southall but the decision not to open a new contact centre makes sense given the number of call centres and seats we already have available.’
Michael Walker, Unison’s London regional officer, said: ‘It will be a matter of sitting down with management to ensure the NHS staff retains as many staff as possible.’