The Office of Fair Trading has cleared NTL's $6bn (£3.4bn) takeover of rival cable TV operator Telewest.
NTL and Telewest plan to take on BSkyB in the UK
It said in a statement that there was justification in referring the deal to the Competition Commission.
The OFT also rubber-stamped satellite broadcaster BSkyB's £211m purchase of internet company Easynet.
The combined NTL and Telewest will have customers in about five million homes across the UK, compared with BSkyB's eight million.
NTL and BSkyB both announced their respective takeovers in October.
The OFT said NTL's and Telewest's local networks did not overlap and they did not compete in providing services over cable.
Analysts had long expected the two companies to come together to better compete against BSkyB.
Both companies are recovering from a downturn, which saw NTL emerge from bankruptcy protection in 2003 and Telewest undergo restructuring.
Although both their operations are focused in the UK, shares in both companies are traded in New York.
NTL is now attempting to buy Virgin Mobile which rejected its initial £817m bid earlier in December.
BSkyB's purchase of Easynet will help it move towards broadcasting some of its programmes via the internet, which many see as the future direction of broadcasting.