Welfare spending over the course of this Parliament has fallen by just £2.5bn despite reforms aimed at saving £19bn, economic forecasters say.
The Institute for Fiscal Studies (IFS) said the reasons included a rise in the cost of pensioner benefits, and an increase in housing benefit spend.
It said Chancellor George Osborne could need to make more spending cuts "just to stay on track".
According to the IFS:
It’s not all about macroeconomic conditions though. The difference between the £1.2 billion cut to DLA spending that was announced and the £1.6 billion increase in spending that is now expected reflects the significant delays to the government’s replacement of DLA with the less generous personal independence payment. And although the introduction of employment and support allowance is not included in the £19 billion of cuts (since it was announced by the previous government), it too has saved much less than expected.