The company provides managed network services to blue chip corporates including ICL Pathway for the UK Post Office Counters' network, CSC/Bae, Next Retail and Powergen as well as a contract to supply broadband Internet to schools in Northern Ireland.
NTL has also been at the forefront of the push towards digital interactive TV and its problems could hit government plans to use digital TV as a key way of e-enabling the UK population.
Digital TV partners include ITN, Universal, BSkyB and a string of top football clubs, which use NTL for their digital TV channels.
Others looking on anxiously include Orange, which last year signed a £150m deal with NTL for the provision of a next generation communications network.
NTL serves 3,242,600 residential customers and businesses in the United Kingdom and Ireland.
The company's troubles follow last week's shock announcement from telecoms provider Energis that it might be in breach of its banking covenants. The company is expected to cut 500 jobs as it attempts to reduce costs.