Morgan Stanley has filed a suit for £7.1m in alleged unpaid fees dating from 1999.
NTL, which employs about 1,000 staff at its UK base in Bartley Wood Business Park, is currently deciding whether to contest the claim, but a spokesman said the group believed it had grounds to object.
The dispute is a surprise, as Morgan Stanley is currently advising NTL on avoiding bankruptcy.
The news comes hard on the heels of Vodafone suing NTL to claim back “excessive annual rent” it paid for renting traditional fixed-line phone lines from the cable firm.
At one point in the late Nineties, NTL was acquiring companies at the rate of one every seven weeks.
The spending spree, including an £8.2billion acquisition from Cable and Wireless, created a £12bn debt.
At one point the company was footing an interest bill of £3.8m a day.
When NTL failed to sell its broadcasting transmission business for £2bn, the company was forced into a debt-for-equity swap.
Bondholders seized control of the business when they agreed to exchange about £7bn of debt for NTL shares.
That averted a collapse but prompted NTL to file for Chapter 11 bankruptcy protection in America, which keeps creditors from demanding payments which they are owed while the company gets back on its feet.
It tried to escape from this last month but failed due to last-minute wrangling between the company and its creditors.
Agreement had been held up by the sheer complexity of NTL’s old capital structure.
The group has 24 classes of bonds, mainly held in American hedge funds, and four different bank loans, and every lender affected had to agree to the deal.
NTL’s year-long £6.6bn debt restructuring is now expected to be completed this month.
Once worth £65, the company’s shares changed hands for less than a penny each last month.
In the middle of November it admitted losing customers as a result of reduced marketing and investment in high-speed internet networks.
Currently Britain’s biggest cable group with 2.9million subscribers, NTL has set aside about £10m to cover the cost of another 250 redundancies — on top of the 8,000 workers already sacked over the last few years.