Fairfax Media and Nine Entertainment are set to officially become one company next month, marking a significant shift in the Australian media landscape.
But former prime minister Paul Keating has described the competition watchdog giving the merger the green light as a "truly appalling decision" which will "poison quality journalism".
The Australian Competition and Consumer Commission ruled on Thursday the $4 billion merger could go ahead.
Fairfax head Greg Hywood told staff soon after the approval would allow the process to be officially completed by December 10.
"The planning and preparation to bring Fairfax and Nine together as one great media company is well under way," Mr Hywood wrote to Fairfax workers in a letter.
Nine has put the date a touch earlier at December 7, but both deadlines depend on the merger successfully passing a series of final hurdles.
They include the proposal's approval by Fairfax shareholders at a November 19 meeting and court approval on November 27.
"The directors of Fairfax unanimously recommend that Fairfax shareholders vote in favour of the scheme, in the absence of a superior proposal," Nine said in a statement on Thursday.
The Australian Competition and Consumer Commission examined more than 1000 submissions as well as documents demanded from Nine and Fairfax before giving the merger the green light.
The authority said the move will likely reduce competition in the Australian news content market - but not enough to break the law.
But Mr Keating said the ACCC's call had consigned key Fairfax mastheads to the "ethical dustbin of Channel Nine".
"What the ACCC has done today is effectively skewer major source media diversity in Australia," the former Labor leader said in a statement.
"A low-rent, news organisation, Channel Nine, will have editorial command of the major print mastheads in the country.
Former prime minister Paul Keating has commented on the Fairfax-Nine merger. Source: AAP
"This will poison quality journalism; but more than that, remove chunks of local specific political issues, normally covered by newspapers, from the political debate."
The merger comes after media ownership laws were changed in October 2017, allowing a proprietor to control more than two-out-of-three platforms - TV, radio or newspaper - in one licensed market.
Mr Keating said those laws has protected capital city print journalism from domination by television companies for 32 years.
Source: SBS News
After Nine and Fairfax proposed the step in July, Communications Minister Mitch Fifield said the law changes had been necessary to give media organisations the best chance of survival.
"The greatest threat to Australian media diversity would be the failure of an Australian media organisation," he said.
Labor communications Michelle Rowland said in September the merger meant the media was about to get "a whole lot more concentrated".
