The contractor, which is building out HOK Sport and Foster and Partners' designs, said the hugely controversial scheme was causing it to lose at least £70 million.
But the Australian firm, which also doubles as a developer, insisted that the project will still be ready for the 2006 FA Cup final next May, a deadline that many observers have claimed should be written off.
Multiplex has been dogged by problems on the Wembley project, which has incurred litigation and higher costs after a change of steel contractor.
The impact of the fixed-price contract was shown in August, as its construction division revealed full-year losses of £26.1 million, compared with profits of £30.8 million in 2004.
The company said today that its exact position on the project was still subject to 'considerable variability' because of the final outcome of steel costs and negotiations in relation to numerous sub-contractor claims.
But a spokesman added: `Multiplex continues to target completion of the Wembley stadium to enable the 2006 FA Cup final to be played, with progressive handover of parts of the stadium expected to commence from 31 January 2006.'