Wembley officials insist the new stadium will be open to stage the FA Cup final in May 2006, despite fresh claims that it is well behind schedule.
GMB union spokesman Kelly Rogers told the BBC her staff "do not believe the stadium will be finished on time".
But Wembley National Stadium Limited chief executive Michael Cunnah dismissed fears it will be delayed.
He said: "Wembley will prove all the doubters wrong - I'm very confident it will be ready in time."
The FA Cup final is scheduled to be played on 13 May 2006 and The Football Association has held talks with the Millennium Stadium on staging the final if redevelopment is delayed.
Artists drawing of the link between the new Wembley and town centre
But Cunnah added: "In all high-profile projects there are so many detractors but there won't be any left after they've all visited the new Wembley. All the detractors will realise that we've built the best stadium in the world.
"Wembley is the most famous stadium name in the world and it needs to stay that way. That's why we're going to deliver something that's worthy of the name Wembley stadium.
"Every time I come on site I'm in awe of this stadium, the size, the scale and the sheer beauty of it. I'm so excited about being here for the cup final in a year's time that I don't try and think about it.
"Not only do we want this to look the best stadium in the world, we also want it to be the best experience for all football fans."
But union official Rogers warned: "We have numerous members that work on the site, particularly on the mechanical side in relation to the roof. That should have been started in October last year.
"It is six months behind so the information we are getting back from our members constructing and building the site is that they firmly believe that it won't be completed by that day.
"I have not had one member come back and say the job will be completed on time."