Yahoo and Microsoft have extended the deadline to finalise their search and advertising deal, announced in July.
Yahoo has told the Securities and Exchange Commission (SEC) that the two companies need more time to finalise the details of the deal.
The companies had promised to complete the agreement by 27 October, but now need more time because of the "complex nature" of the transaction, said reports.
Yahoo and Microsoft claim good progress has been made and say they are working to finalise the agreements as "expeditiously as possible", according to the SEC filing.
But no new deadline for reaching a final settlement has been set.
A high level of scrutiny from US anti-competition authorities could be delaying the agreement as both parties struggle to find an acceptable way of structuring the deal.
Regulators blocked Yahoo's proposed deal with Google, but the deal with Microsoft joins two much smaller companies and was expected to get easy approval.
But anti-trust experts have warned that US regulators sometimes oppose mergers between number two and three players.
US authorities have objected to the formation of duopolies in the past because of concerns of collusion to block all other competition.