A £21million accounting mistake in Hertfordshire County Council's annual finances has been blamed on a “presentational error”.

Auditors looking through the county's books for 2009/10 found its general fund was £16.2million in surplus, although finance officers had expected a deficit of £4.5million.

The auditor later discovered a figure referring to a fixed asset, such as a building, had been double counted, leading to the mistake.

As a result, the county's finances will now not be signed off until the end of this month – four weeks late.

A Hertfordshire County Council representative said: “The £21m over-estimation on our draft accounts resulted from a presentational error, where a figure was placed in the wrong area of the balance sheet.

“This has no financial consequence – it does not impact on council tax. The Audit Commission identified the source of this error and we are currently working with the auditors to correct this.

“Our final accounts will be published and audited by the end of October in agreement with the Audit Commission.”

Meanwhile, the Green Party said the council's accounting was “not good enough”.

Councillor Ian Brandon, who represents Callowland Leggatts, said: “In the particular financial downturn we are currently facing, it becomes increasingly important, both as a local authority and to its taxpayers, that the council is accurate in its accounting.”