The announcement is expected tomorrow when the radio station owner reports its half-year results.
It follows a warning in March that annual profits will be down at least 10% year-on-year, largely because of the absence of dotcom advertising, which was particularly prevalent in London.
The station has also been hit by the general slowdown in advertising and a drop in audience figures, which could undermine station's advertising rates.
Recent Rajar figures show its listening hours are falling 4.8% year-on-year.
The group is expected to make full-year pre-tax profits of between £30m and £32m, compared with March's forecast of £35m and pre-tax profits of £40m a year earlier.
Capital's problems mushroomed this week when one of its most popular DJs, Steve Penk, quit for rival Virgin Radio. Penk, who is renowned for making a spoof phone call to Tony Blair pretending to be William Hague and other shock tactics, will host a drive-time show on Virgin.
Capital shares were trading at 690p this morning, a far cry from last March when it peaked at 1,900p. The group's shares have under-performed the media sector by 22% this year.