Yahoo has revealed that its search deal with Microsoft has been delayed while the pair iron out a number of undisclosed details.

The two firms signed a deal in July in which Microsoft's Bing technology will power Yahoo search, while Yahoo will use its solid relationship with advertisers to become the 'salesforce' for both companies' search advertisers.

The pair said at the time that they expected to agree the final terms of the deal by 27 October, but a Yahoo filing with the US Securities and Exchange Commission shows that this has not been the case.

"Given the complex nature of the transaction, there remain some details to be finalised," Yahoo said in the filing. "The parties are working diligently on finalising the agreements, have made good progress to date, and have agreed to execute the agreements as expeditiously as possible."

Commentators have suggested that the delay is probably due to the extensive terms of the agreement, and the fact that the contract will run for 10 years.

Once Microsoft and Yahoo have gained regulatory approval to partner in the US, they will then have to face European regulators.

This could prove an even bigger hurdle, considering the time it is taking European authorities to sanction Oracle's proposed acquisition of Sun Microsystems.

Microsoft did not immediately respond to requests for comment.

Yahoo said in a statement: "Given the complex nature of this transaction there remain some issues that need some additional clarity and definitive details. Both companies are optimistic that we will be able to close this deal by early 2010."