According to sources close to the scheme, the extra cash will be needed to pay for the cladding on the waterfront buildings and to 'finish off' the main public piazza.
Work on the project, which will create a 10,000-seat arena, conference centre and exhibition hall, is already well under way, and Liverpool City Council is confident the complex will be completed in time for the city's 2008 Capital of Culture celebrations - despite the funding hiccup.
The council is not the only supporter of the key £146 million development. It is also being bankrolled by Liverpool Vision, the North West Development Agency and English Partnerships, although where the additional cash will come from is not yet clear.
Speaking about the funding shortfall, Frank McKenna, of Downtown Liverpool in Business, said: 'This news is disappointing but not surprising.
'With the experience this country has had with stadium development, be it Wembley or the Commonwealth Stadium, it was almost inevitable that it would go over budget.'
In an attempt to explain where the price hike has come from, Jim Gill, chief executive of Liverpool Vision, said that the initial estimates were never fixed.
He said: 'There are some additional costs, but we always envisaged having them at some stage, as the costs were less certain 18 months ago than now.
'We are very happy with the progress overall and there is no panic about the costs.'
A spokesman for Wilkinson Eyre said: 'We can confirm that the application for additional monies is to fund a series of client changes, enhancements and the client's fit-out.'