Leaked internal documents have revealed that the government could breach its self-imposed cap on welfare spending as a result of the cost of the main sickness benefit.
The memos suggest Employment and Support Allowance (ESA) costs are rising with few cost-cutting options.
In March, MPs agreed a 2015-16 welfare cap of £119.5bn, excluding the state pension and some unemployment benefits. The Treasury said it was "confident" of remaining within the cap, saying delivery of ESA was "back on track".