It is understood the Ministry of Commerce of the People's Republic of China (Mofcom) has yet to accept formal notification of the combination, although discussions opened in August.
From the point it accepts formal notification, it is believed Mofcom has up to 120 days to make a decision.
Aegis claimed that it and Dentsu "remain confident that the scheme [ie the transfer of ownership from Aegis shareholders to Dentsu] will become effective on or prior to 28 February 2013", and that it and Dentsu "remain in constructive dialogue with Mofcom".
It is understood that while Aegis and Dentsu are reluctant to proceed without clearance in China, the issue is not a threat to their £3.2bn deal.
China is the third-largest market in the world for advertising spend and is predicted to overtake Dentsu's home market of Japan in the next few years.
China and Japan are currently at diplomatic loggerheads over their rival claims to a group of islands, known as the Senkaku in Japan and as the Diaoyu in China. Japan's acquisition of some of the islands from their private Japanese owners in September sparked a boycott of Japanese-made goods by Chinese consumers.
Aegis remains listed on the London Stock Exchange until the scheme becomes effective. Its share price was unchanged in early trading from the 235p level it has been at since the deal was agreed in July.
All other national and regional competition authorities have given the green light to the combination.
Separately Aegis has acquired IQ mobile, a mobile agency based in Austria.
Nigel Morris, CEO, Aegis Media Americas & EMEA said: "The acquisition of IQ mobile, a highly regarded specialist in mobile marketing in the Austrian market, will be important in differentiating Aegis Media’s digital service offering in that market.
"This acquisition is in line with our strategy to strengthen our digital capability and will provide us with opportunities to increase Aegis Media’s mobile business in Austria and across Central and Eastern Europe."Follow @DanFareyJones