The extraordinary admission means that the hugely controversial scheme will not be able to hold a 'test event' until June 2007, making it nearly two years late and missing the cup final in May.
The stadium was originally due to open in August 2005, but Australian construction firm Multiplex announced a series of delays which has forced the Football Association to make changes to its schedule.
The client, Wembley National Stadium Limited (WNSL), stated that it remained confident the rebuilt stadium would be complete by the end of the year, even if Multiplex missed a previously promised September deadline.
But Multiplex said the likely completion date had now been pushed back because WNSL had not yet started work for which it was responsible.
The Multiplex statement read: 'WNSL is responsible for obtaining approvals from various authorities in order to ensure the stadium is fully operational for events.
'To obtain these approvals a series of test events will be required at the stadium to demonstrate that the integration of the safety and control facilities are working properly, so that a 90,000 spectator event can take place.
'Unless and until WNSL hold and successfully complete this series of test events, and obtain the required approvals, Multiplex will be prevented from achieving practical completion.'
Multiplex said it consulted an expert in the operation of major stadia who indicated it was unlikely that the stadium will be able to hold a test event before June 2007.
Construction work remains to be done on drainage, the roof and building-management systems.