If you're using the model financial regulations, 4.2 states:
4.2. No expenditure may be authorised that will exceed the amount provided in the revenue budget for that class of expenditure other than by resolution of the council, or duly delegated committee. During the budget year and with the approval of council having considered fully the implications for public services, unspent and available amounts may be moved to other budget headings or to an earmarked reserve as appropriate (‘virement’).
It's also worth checking what limit your council has chosen in 4.5 for emergencies.