In a surprise move, district auditor John Magill issued a revised calculation last week which puts the total surcharge at nearly £30m.
The previous figure represented the estimated loss between 1987 and 1989 resulting from the council's controversial designated sales policy.
The revised figure, which is provisional, adds estimated losses between 1989 and 1994.
The decision to increase the surcharge was based on submissions from objectors to the sales policy at the public hearing on Mr Magill's provisional findings.
Accountants, hired by Dame Shirley, had argued that designated sales did not result in any loss to the council.
In issuing the revised calculation, Mr Magill appears to have rejected claims put forward by the group facing surcharge that the recent Law Lords ruling that councils were only required to provide temporary accommodation for the homeless would significantly reduce the level of surcharge (LGC, 14 July).
'We are pleased that John Magill has so thoroughly rejected the arguments put forward by Shirley Porter and her accountancy witnesses at the audit hearing,' said a spokesman for the group of 12 objectors who made the original complaint.
Supporters of those facing surcharge said Mr Magill's decision showed his unfitness to continue.