The OFT referred the merger to the Competition Commission and said the deal would 'create a monopoly supplier in a sector where it is difficult for companies to enter the market'.
It added that the deal would 'reduce the overall innovation, quality and price benefits enjoyed by broadcasters and ultimately consumers'.
NGW owns and operates two Freeview digital TV multiplexes and operates the BBC's transmitters and a network of more than 5,000 mobile communications sites around the country.
Australian banking group Macquarie acquired Arqiva (then NTL Broadcast) in 2004. Arqiva operates around 2,500 TV and radio transmitters, including ITV's transmitters and Digital One, the national commercial digital radio multiplex.
The Competition Commission will conclude its enquiry by January 2008. The most extreme scenario would see it block the merger, forcing NGW to look for another buyer.
Arqiva has argued that 'a single, strong and efficient organisation would be in a better position to support the delivery of all-digital transmission services once analogue television is turned off'.