London black cabs took centre stage at the Olympics closing ceremony when five Spice Girls gyrated on top of them, but their manufacturer, Manganese Bronze, has since come crashing down to earth after discovering accounting errors estimated at £3.9m.

The group admitted on Tuesday it would have to delay its half-year results, due next Monday, for around a month after bosses discovered the understatement of losses, which go back over several years.

The company, which is 20% owned by Chinese car group Geely, blamed an IT system introduced two years ago along with "procedural errors" which meant that a number of transactions relating to 2010 and 2011 were not properly processed.

The company's shares, traded on London's junior Aim market, lost a third of their value.

Manganese Bronze has had a rough ride over the last few years, and it remains in the red. It now expects a net loss for the first half that is "substantially" higher than the £700,000 loss reported a year earlier. This means it might miss market expectations for the whole year. Broker MC Peat & Co had pencilled in a £1m profit in March.

Manganese Bronze made 1,500 black cabs for the UK at its Coventry factory last year, with another 705 purple taxis manufactured in China for the international market.

After winning an order to deliver 1,000 taxis to Azerbaijan in March 2011, problems in securing finance due to the eurozone crisis delayed the dispatch until January. The delay along with a drop in UK orders pushed the taxi maker into a £2.6m loss for 2011, albeit down from £6.3m the year before.