They argue that cuts to benefits like Personal Independence Payments (PIP) would push 700,000 disabled households into poverty, rather than encouraging employment.
The charities dispute the idea that cutting benefits incentivises work and instead are calling for better support systems to help disabled people find jobs.
This comes ahead of Reeves' spring statement on March 26, where welfare reform is expected to be a target for savings.
The government maintains that its plans will support disabled people into work and create a more sustainable welfare system.