The chancellor will announce a £500m package to support mental health services in England after increased demand for support during the pandemic.
The Treasury said the majority of the funding will be spent on specialist services for young people, including in schools, and support for NHS workers.
Mr Sunak is also expected to vow for rapid progress to tackle the backlog of adult mental health referrals.
He will pledge the new funding in Wednesday’s Spending Review.
In the same speech, the chancellor will unveil his long-term plan for infrastructure investment.
While the funding only applies to England, the devolved administrations in Scotland, Wales and Northern Ireland will