If there is one thing you can be sure of when it comes to Lord David Frost, it is that his incredible passion for Brexit will not be dimmed by the huge adverse effects of this folly.

The man who was Boris Johnson’s chief Brexit negotiator gave evidence this week to the House of Commons Business and Trade Committee and, even though you always expect something a bit jaw-dropping from Lord Frost, it was quite fascinating.

Committee chairman and Labour MP Liam Byrne presented Lord Frost with a raft of evidence which would surely convince anyone without some kind of ideological entrenchment that Brexit has worked out very badly indeed.

However, Lord Frost was having none of it.

As is usual with the Brexiters, his testimony seemed very heavy indeed on excuses.

A change of administration in the US was flagged as a reason for the failure to achieve a much-vaunted transatlantic trade deal, and for falling short of a key target to have 80% of UK trade covered by free trade agreements. However, according to Lord Frost, a return of Donald Trump as US President might help, although the former chief Brexit negotiator conceded he could not say for sure.

It is not, apparently, appropriate to extrapolate pre-Brexit goods exports growth to the post-departure period to determine where the UK might be had it not left the European Union, according to Lord Frost.

When it was put to him that Brexit had led to small and medium sized enterprises, especially in high-growth sectors, withdrawing from export markets, Lord Frost declared: “A lot of the evidence is anecdotal, of course, rather than reliable.”

And, oh, Lord Frost does not accept the independent Office for Budget Responsibility’s assessment that the cost of Brexit to the UK is around 4% of gross domestic product.

This OBR view was flagged by the body’s chairman, Richard Hughes, when he told the BBC’s Sunday with Laura Kuenssberg programme last spring: “We think that in the long run it reduces our overall output by around 4% compared with had we remained in the EU.”

Sections of Lord Frost’s testimony very much chimed with that given to the same committee by Secretary of State for Business and Trade Kemi Badenoch in January.

However, while not wishing to diminish Ms Badenoch’s great enthusiasm for Brexit, Lord Frost’s passion for the cause seemed so much greater.

Mr Byrne kicked off the questions at Tuesday’s session on “export-led growth” by asking Lord Frost: “Are you pleased with the way trade has progressed since Brexit?”

No prizes for guessing the response.

Lord Frost, coming across as a true believer, replied: “Yes, I think is the short answer, because I think all those who predicted a dramatic change in our trade and a dramatic turn for the worse have been proved wrong.”

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And even when he conceded that Brexit had damaged UK export volumes, he was entirely unrepentant.

Mr Byrne put it to Lord Frost that goods exports to Europe between 2014 and 2018 were growing at nearly 3% (taking the average per annum increase) and that, “obviously if goods exports had carried on growing at that level from say 2021 we would have had about £54 billion worth of extra exports of goods into Europe”.

The committee chairman added: “That would seem to outweigh the gains that have been listed against the free trade agreements, which looking across Japan, Australia, New Zealand, CPTPP (the Comprehensive and Progressive Agreement for Trans-Pacific Partnership) is about £784 million. So that’s an order of magnitude lower than what our goods exports may have been if goods exports carried on growing at the rate before Brexit.”

Lord Frost remained completely unflustered in the face of these two hugely different numbers, which surely throw into stark relief the foolishness of Brexit.

He said: “Well, I think we have all learned in the last few years about the risks of simple extrapolation from previous trends. And your base period is one when we were still coming out of the financial crash. These figures are all extremely sensitive to the base years that you choose.

“I repeat I would say and to be clear on this I think there has been a very small effect from leaving the single market and the customs union to our goods trade – maybe 2%, maybe 3% ¬- but services trade has gone up hugely, let’s not forget that’s about half of our trade and, secondly, trade and growth are not the same things.”

Lord Frost added: “And I would disagree with those who say there is a mechanical relationship between openness, between exports, between trade levels and growth, and I think the last few years have borne that out.”

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It was becoming clear, as you might have expected, that Lord Frost was not going to concede that Brexit is damaging the UK economy on the scale that the OBR and other experts say it is, although it is perhaps interesting that he did concede goods trade had been hit to an extent.

Lord Frost was then asked by Mr Byrne if he believed that, if the supply-side reforms suggested by the former chief Brexit negotiator were pushed through, “actually that would capitalise on the freedoms that we do now have outside the EU”.

The peer replied: “I do. And as I said my view is always that there was a very slight hit to leaving the single market and customs union but that should be well compensated for by the ability to do structural reforms in the domestic sector.

“Now unfortunately we haven’t done as many of those as I would wish but if you look at our productivity growth record over the last couple of decades it has got gradually worse. That is because our domestic economy has been furring up the arteries and we need to liberalise, we need to reform, we need to do a lot of things to bring growth back. That’s now in our hands.”

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Asked if there were particular things he would put at the top of his long list of proposed reforms, Lord Frost replied: “Where to start? Of course, a lot of these things have a mix of what would have been purely domestic policy and EU policy. I would certainly put the current version of net zero and high energy costs at the top of that list. I would want to see a lot more housebuilding and changing to planning regulation. I think we need more broadly to change our economic model so it is less reliant on immigration and of course we need to get tax and spending down because there is no future for the country as a high-tax, high-spend, social democrat economy.”

It was difficult to see how Brexit even related to much of the ideology that Lord Frost was expounding.

He talked, in a fairly broad-brush way, about employment law, environment law and planning law having been “influenced by EU regulations and doctrines”.

The case that Lord Frost made for Brexit might have been stated with true belief but it was, to put it mildly, monumentally unconvincing.