Childline, which is operated by the NSPCC, has made its night service a priority for 2008 and 2009 and will hire additional counsellors in Birmingham and Glasgow to answer calls from the autumn.
Research shows that night-time callers often ring in a state of distress and need emergency help.
BT, which has supported Childline since its launch 22 years ago, said it would donate 5% of the revenue it receives from searches completed by clicking on a sponsored link.
The arrangement runs from today onwards. Asked when it would end BT said only that it was "aiming to build its charitable efforts for ChildLine into its business process to ensure a long-term, sustainable commitment to the charity".
Esther Rantzen, founder of Childline, said: "As people are increasingly spending more time online, this is a fantastic way to generate funds for a vital lifeline for children and young people across the UK."