The money should help Britain offset an economic slowdown caused by uncertainty over Brexit.
Britain will have a £26.6-billion ($35.2-billion, 31.1-billion-euro) war chest to battle the potential damage of its exit from the European Union, a government oversight body said.
With the Brexit deadline approaching on March 29 and no deal reached over the UK's departure, the government is stepping up economic contingency plans.
Its efforts will be helped by extra money if it keeps to a budget deficit below two percent of gross domestic product in 2020-2021, the Office of Budget Responsibility, which the government tasks with reviewing its budgets, said late Wednesday.
Its report assessed finance minister Philip Hammond's latest economic report to parliament, which he delivered earlier Wednesday.
"The forecast changes and policy decisions leave (Hammond) with £26.6 billion (1.2 per cent of GDP) of headroom against his fiscal mandate," the OBR said.
"This is up from £15.4 billion in October, as the fiscal costs of the temporary near-term cyclical weakness of the economy have been swamped by the fiscal gains from higher income tax and lower debt interest spending."
The figure represents the difference between the OBR's new budget deficit forecast and the government's own fiscal targets.
The money should help Britain offset an economic slowdown caused by uncertainty over how Brexit will be resolved.
British Prime Minister Theresa May stressed on Tuesday that parliament's approval of the withdrawal agreement she has reached with Brussels will deliver a fiscal "Brexit dividend".
But lawmakers still rejected her deal, sending it to a second defeat since January.
May on Wednesday agreed to ask the 27 EU leaders for a postponement of the Brexit deadline.
But she also warned that unless parliament drops its opposition, any Brexit extension could be long.
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