Thousands of jobs at Virgin Media O2 are at risk as the telecoms company prepares cost-cutting measures amid heavy debts and intensifying competition. The number of redundancies has not been finalised, but the cuts are expected to affect a substantial proportion of its 16,000-strong workforce.

Most affected posts are expected to be in non-customer-facing roles. Senior leadership changes are already under way, while the company is also expected to review its use of contractors and suppliers. Virgin Media O2 declined to comment.

Prospect, which represents staff, described the prospect of cuts as disappointing and deeply unsettling. Its deputy general secretary, Noel McClean, said the union would support members and work with employers to reduce redundancies, secure retraining and find opportunities for redeployment.

The planned cuts come alongside a potentially significant change in the company’s broadband investment strategy. Virgin Media O2 is understood to be close to agreeing a deal to use BT’s fibre network and is considered highly likely to announce an end to all fibre investment in its results on Tuesday. Neither development has yet been announced.

The company had reportedly halted upgrades and expansion of its own infrastructure to ease concerns about its borrowing. Its latest accounts show total debt of £27.2bn and assets worth £45.9bn as at 30 June. BT shares rose 7% on Friday following reports of the prospective network agreement.

Virgin Media O2 faces competition both from BT and from cheaper alternative network providers, including Hyperoptic, Community Fibre, Gigaclear and YouFibre. It sought to respond through a proposed £2bn purchase of Netomnia via nexfibre, a joint venture owned by Virgin Media O2’s owners. The competition regulator has provisionally identified concerns about that deal, with a final decision due in December, prompting anger from Virgin Media O2.

The company has also firmly opposed BT’s agreed takeover of TalkTalk, arguing that it would strengthen BT’s hold on the market. BT says the acquisition protects TalkTalk customers because their provider faced collapse without it.

For staff, the immediate uncertainty is how many posts will disappear and whether retraining or redeployment can limit compulsory redundancies. The final scale of the cuts remains unsettled.