Most at risk is the night helpline service, which relies on paid staff rather than volunteers. They counsel fewer children, but often those who are in most need.
Esther Rantzen, ChildLine's founder and chair, warned: "Many of the desperate children we speak to overnight are facing real emergencies and don't know where else to turn for help."
Compassion fatigue is being blamed for donations drying up. Since the outpouring of generosity following the tsunami disaster earlier this year, gifts to ChildLine have slumped. Now the charity has launched a mass appeal to keep its services open.
In Scotland, around 30 calls a night are classed as warranting immediate intervention from other agencies such as social services or police. A third of those calling are aged 12-15.
"During the night between 2am and 6am, sometimes there can be single-figure calls, but these are clearly the most vulnerable," Anne Houston, director of ChildLine Scotland, says. "A lot of these children are not ready, because of lack of confidence or trust, to call other agencies. They often need to talk to ChildLine first.
"My main concern is that if there is no night service, it could have dire consequences for the most needy ones."
Further cuts have not been ruled out, although the Chips service (ChildLine in Partnership with Schools) will continue.
Donations can be made by calling 0800 876 6000 or at www.childline.org.uk.