Interserve has been forced to write off £11m, largely because of costs associated with an investigation into accounting irregularities at the company.

The support services company payed £8m for the investigation into “accounting misstatements” at its industrial services division, which came to light last September.

It was also hit by a one-off cost of £4m associated with the integration of facilities management company MacLellan which it bought last July.

Interserve added that it had made £1m from a PFI asset sale in January 2006 and that it had secured contracts and preferred bidder appointments on work worth more £1.8bn.