Electricians were due to meet management on Wednesday to discuss a new offer, but site sources said this was likely to be rejected and strike action was expected.
Electricians at the opera house, who are already operating an overtime ban, want an extra £5 an hour on top of their £7.88 national rate because they believe they are not being treated as well as other electricians in London.
Electricians at the Millennium Dome receive £3 an hour above the national rate, and the opera house electricians want at least pay parity with them.
However, Royal Opera House M&E contractor Balfour Kilpatrick is refusing to pay anywhere near this rate, although it may relax conditions governing bonus payments. Until now, it has penalised electricians if they were even one or two minutes late for a shift.
Balfour Kilpatrick was also expected to offer new incentives for weekend working, which has been a casualty of the overtime ban.
The electricians' most powerful bargaining tool until now has been the assumed deadline of 1 December. But ministers have recently suggested that there is some flexibility about this, undermining the electricians' position.
The electricians think this is a millennium project, but it is not. They should realise there is no obligation to open before the new year
A minister was quoted this week in the Financial Times as saying: "The electricians think this is a millennium project, but it is not. They are flexing their muscles, but they should realise there is no obligation to open before the new year." Project sources still believe the project will finish before 1 December, but the scheme has been re-programmed several times to allow for delays.
It is understood that work planned for completion in September, as well as some plans to test scenery equipment, have been put back to November to allow electricians to make up lost time.
But the main auditorium is almost complete and it is possible that the opera house will open on time but with many behind-the-scene areas unfinished.
The government also insisted this week there would be no extra money for the scheme in addition to the £78m lottery grant already awarded.
However, the project is almost certain to cost more than the £220m budgeted for. Balfour Kilpatrick has put in a £10m-15m claim, with a host of other trade contractors submitting smaller claims.
One site source said: "The situation will get worse before it gets better. We are all concerned, actually. I think there is no chance we will accept what is on offer." Electricians union the AEEU, which was unavailable for comment, has been attempting to broker a deal between Balfour and the electricians.