Carillion uncovers more problems on big Mowlem contracts including Dublin port tunnel and Exeter schools projects.
Carillion said that it had already written down £45m on "a small number of larger contracts" but that the position on those contracts had since deteriorated.
Mowlem's Dublin port tunnel and Exeter schools projects were particularly problematic according to Carillion.
Carillion has estimated that average net debt in 2006 will be £200m, peaking at £250m, but that it would reduce to below £100m by the end of 2007.
The contractor also said that by the end of 2007 cost savings resulting from the acquisition were expected to reach £23m a year.