Delays have pushed back the project which was originally supposed to be finished by September 2006 at a cost of £8.6m.
The Commons Commission said last year it would open in "early 2008" but now says trials must be completed first.
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MP Nick Harvey, the Commission's Spokesman, said: "The date for full opening of the building will be fixed after trials to test the technical equipment and the handling of visitors have been completed.
"Some trials have already occurred, and more are taking place at present.
"The project was originally planned on the basis of an opening date of autumn 2006 but delays were caused by unexpected problems with piling work and with services buried beneath the site, and later by difficulties with the construction of the building itself."
(CD/JM)
