A "brief delay" has been announced for the search advertising partnership between Google and Yahoo!.
Google chief executive Eric Schmidt had previously said that the agreement was likely to come into force during early October.
However, this has now been postponed for an unspecified time period while the US authorities look into the antitrust implications of the deal, which will see Google ads shown next to Yahoo!’s search results for specific terms.
Since the agreement was unveiled, worries have been raised that it could result in higher pay per click marketing costs and a less competitive market in general.
The deal was first announced in June and SearchIgnite released a report the next month indicating that the contract could bring higher keyword ad prices for the majority of marketers using Yahoo!.
Yahoo! said in a statement: "We have had discussions with regulators and look forward to responding to their questions about this agreement."