C&W expected revenue for the six-month period ending 30 September in its global division - excluding network capacity sales - to drop by 6%, against a previous forecast of 0% to 10% growth.
With the drop in revenue, the global division is expected to report a half-year loss, before interest, tax, depreciation and other one-off items (including charges relating to its discontinued US retail voice business), of about £120m.
"There will be further reductions in cost and capital expenditures, therefore you can't rule out job reductions," said C&W spokesman Peter Eustace, "but we can't provide any details on how many jobs may go or which sectors may be affected. There will have to be consultations about any developments with employees first."
C&W's global division, which supplies telecommunication services to companies in the US, Europe and Japan, employs about 12,000 people.
The company's service provider business is suffering as a result of competitive pricing in the Internet protocol transit market and customers withdrawing from the carrier market.
On Monday, C&W announced plans to sell its US retail voice customer base to a subsidiary of Primus Telecommunications Group for $32m in cash.
In the UK, revenue in C&W's business markets is falling, although its unit serving enterprise customers has shown steady growth.
The company's half-year results will be announced on 13 November.