The report warned that this figure would rise as contractors lodged claims for costs incurred during the delays, which were caused by London Underground handing over the land above Westminster Station 10 months late. Contractors have already been paid £3.1m for the delay.
This is the latest in a series of cost overruns on the Michael Hopkins-designed MPs building, which opened last September, and is the most expensive office block in Britain.
But mystery surrounds how many contractors are claiming and how big those claims will be.
Portcullis House project manager Andy Makepeace refused to comment, saying it was a commercial matter. Construction manager Laing said it was not making a claim.
The Commons was hit by a £9.9m bill after unsuccessfully fighting a claim from US contractor Harmon. The firm sued after UK-German joint venture Seele Alvis won a £233m cladding and fenestration contract instead of Harmon, whose bid was £2m cheaper.
The House spent £4.6m on legal fees and also paid Harmon £5.26m in an out-of-court settlement.
One source close to the project said this week: "It was an horrendous amount of money and an expensive mistake."
