The forecaster said proposed tax cuts would be partly financed by some reductions to spending as well as a fall in fiscal headroom, the state’s buffer to meet fiscal rules, from about £13 billion to £8.9bn.

But he stressed that the Budget “only just” met government fiscal targets to get debt to start to fall as a share of gross domestic product within five years and still faces “significant risks.”

The OBR said the fiscal headroom would be halved if the government ultimately freezes fuel duty, as it has done every year for the past decade, and would be wiped out if the government meets a commitment to increase defence spending to 2.5 per cent of GDP.

The forecaster said this could be offset by spending cuts, but that it does not yet know where these might be made.

Mr Hughes said: “They have a workforce plan for the NHS, they have commitment to grow defence spending in line with GDP, they have commitments to grow overseas aid spending in line with GDP.

“They have ambitions in some areas to go even further.

“How they make that add up with the rest of Whitehall departments, we don’t know.”