Chancellor told to guard against march to “economic nationalism” under banner of Brexit

She said the country must “retain the UK’s historic commitment to openness, to trade, to investment and to migration”.

“Global Britain must be a reality and not just a slogan,” she said.

“The economic case for leaving the EU has always rested and continues to rest on openness and we must not allow the Brexit process to mark the start of a descent into economic nationalism.

“It is only through productivity growth that the Chancellor has any hope of meeting the demands for additional spending on welfare, social care, prisons, the NHS, public sector pay and Brexit contingency measures without damaging the Government’s hard-won reputation for fiscal credibility.”

Ms Morgan, a strong Remainer, took a sideswipe at those Brexiteers in her party who had promised a “pot of gold” under the Brexit rainbow.

She said: “For the avoidance of doubt let us for the thousandth time dismiss the idea that a Brexit-induced fiscal windfall will relieve the pressures on our health service.

“There are no easy choices and there is no pot of gold under the Brexit rainbow. Those who persist with this myth may win short-term approval from certain quarters of the media but at the cost of long-term damage to trust in politics.”

Ms Morgan welcomed the Chancellor’s announcements in the Budget on technology investment, tax and his handling of the public finances.

The Loughborough MP also reserved judgement on Labour leader Jeremy Corbyn’s response, saying: “At this point in the debate my predecessor as chair of the Treasury Select Committee would always congratulate the leader of the opposition on making the most difficult speech of the parliamentary year.

“I am happy to continue that tradition although I have to say that much of his speech was pre-written and I’ll suspect he’ll want to look at more of the detail and find more to welcome.”