* Investigation to take 180 days, could be extended by 120 days
* Initial probe showed deal might have violated antitrust law
* Deal to be blocked if market share found to be over 50 percent (Adds details and background)
HANOI, May 18 (Reuters) - Vietnam's Ministry of Industry and Trade said on Friday it had launched an in-depth investigation into ride-hailing company Grab's takeover of Uber Technologies' Southeast Asia business after an initial probe found it might breach antitrust law.
The investigation will take 180 days and could be extended by up to 120 days, the ministry said in a statement.
The ministry said earlier this week its initial probe suggested the deal might have violated antitrust law
It warned last month the transaction could be blocked if Uber and Grab's combined market share in Vietnam exceeded 50 percent, the maximum allowed in the country.
Uber and Grab announced a deal in March under which Uber would take a 27.5 percent stake in Grab in exchange for its Southeast Asian business.
The U.S. company had previously sold operations in China and Russia to local rivals.
Uber and Grab did not immediately respond to Reuters' requests for comments. (Reporting by Khanh Vu; Editing by Keith Weir and Mark Potter)