The admission was made in a confidential letter, obtained by Building, that was sent by Balfour Kilpatrick project director John Moore to opera house construction manager Schal last week.
The news comes in the week that 250 electricians and pipe-fitters working for Balfour Kilpatrick walked off site in protest at safety standards.
In the letter, dated 17 March, Moore tells Schal project director Paul Reeder that Balfour Kilpatrick requires an extension to its contract to 31 January 2000, two months past the opera house opening date of 1 December 1999.
The letter lists a number of areas where Balfour Kilpatrick has been unable to carry out electrical work because of earlier delays.
The Floral Hall, the giant glass structure that will serve as a foyer to the building, is causing the biggest problems.
Further extension requests
The letter says: "We refer to our letter dated 10 December 1998, which recorded an extension of time request for Floral Hall.
"Unfortunately we regret to record that as a result of further delays beyond our control a further extension of time is required until 31 January 2000.
"You have been notified of these delays in various correspondence, reports and notices already submitted to you. We will continue to advise you of problems if, and when, they arise."
Project insiders said the delays were caused by Schal's failure to ensure that areas scheduled for M&E work were ready.
The letter continues: "As previously stated the revised completion date has been derived from an analysis of the original contractual commencement date, the effective commencement date and the period of delay currently recorded against the July 1998 accelerated recovery programmes.
"We are continuing to examine our original contractual entitlements and will advise you of any findings which effect the foregoing method of calculation.
"Detailed information in support of these circumstances has again already been provided to you in our correspondence, reports and notices. Further particulars will be provided as and when they become available."
The letter adds that Schal has also failed to reach agreement with Balfour Kilpatrick for completion of other areas of work and warns that the lack of agreement could cause further delays.
Completion dates not set
The areas that still have no completion programme include the fly-tower, work inside the opera house auditorium, plant rooms and side and rear stages.
Russell Street and James Street completion programmes, a mechanised scenery moving system and staircase works are also threatened by delay.
The letter says: "If and when programmes for these areas are … agreed and the completion dates therein for our works exceed the current requests for extensions of time … we will have to submit further applications for extensions of time."
Balfour Kilpatrick and Schal refused to comment on the letter. However, a spokesman for Schal said: "The Floral Hall and the Royal Opera House redevelopment remain on target to open in time for the first performance in December."
Insiders confirmed that parts of the project were running late but said that Balfour Kilpatrick was using the threat of working beyond the opening date to reinforce claims on the project.
It is understood that Balfour Kilpatrick is pressing for about £15m in addition to its £12m contract sum.