Until relatively recently, Lord Frost was little known beyond Westminster and Whitehall. But after Mr Johnson appointed him Chief Brexit Negotiator in July 2019, he became a household name. Almost overnight, Lord Frost toughened up the UK’s negotiating stance, before striking an acceptable exit deal with Brussels – so ending a prolonged period of tortuous Parliamentary gridlock. A delighted Johnson ennobled his right-hand man, bringing him into the Cabinet. “He’s the greatest Frost,” quipped Mr Johnson, “since the Great Frost of 1709.”

In his pre-Christmas resignation letter, Lord Frost acknowledged it was Mr Johnson’s “outstanding leadership” that ensured the 2016 Brexit referendum was finally implemented – “a moment of grave constitutional crisis”. He praised the “stunning election victory” the Prime Minister secured in December 2019.

But Lord Frost was quitting the Government, his letter explained, due to “concerns about the current direction of travel”. And now, just weeks on, the Prime Minister’s former confidante is becoming more vocal, spelling out his concerns, urging Mr Johnson to follow a path of “low taxes, free markets and free debate” to revitalise the UK.

“There has always been a battle within the Conservative Party between a free-market, low-tax approach and a more consensual, ‘Macmillanist’ view,” says Lord Frost, in an exclusive Telegraph interview. “I think history shows that the best way of producing prosperity is free markets and encouraging free individuals to pursue their own lives.”

In April, a slew of pre-announced tax increases kicks in, including higher national insurance contributions and a freezing of basic- and higher-rate thresholds. Lord Frost’s opposition to these rises – coinciding as they do with spiralling fuel bills and a broader cost-of-living squeeze – chimes with that of many Tories, at Westminster and beyond.

“I don’t think Conservatives should be raising taxes – and one of the difficult decisions we should have taken is not to do so,” says Lord Frost. “We should be pursuing policies that increase growth, while controlling spending, getting things in order after the massive economic shock of the last couple of years.” The Prime Minister “gets the case for low taxes”, Lord Frost insists. “But you’ve got to have the discipline and willingness to see things through,” he adds – suggesting that, in his view, Mr Johnson does not.

Lord Frost is similarly critical of the Government’s environmental agenda, particularly the “net-zero” commitment to limit carbon emissions by 2050. Again, this is a policy area that worries many Tory backbenchers and party activists as well, given the high levies charged on household and commercial energy bills that are channelled towards renewable energy.

“I accept climate change is a problem – I just don’t think it’s necessarily the most significant problem the country faces at the moment,” says Lord Frost. “And I feel we are rushing at some of this stuff, bringing in unnecessary measures too soon.”

The Government is misguided in “trying to pick winners, backing new energy technologies that are unproven and not necessarily the best way forward, paying out subsidies that are funded by increasing costs on individuals”, he says.

“I know respectable leaders around the world must espouse green politics, and if you don’t you are thought to be a bit dodgy, but I would pace it a bit,” he argues. “People have been sold the view you can do net zero without it really costing anything – and that’s obviously not true.”

But Lord Frost saves his most trenchant criticism for the Government’s handling of the pandemic, confirming he left office due to the latest round of anti-Covid restrictions. “I didn’t agree with the Plan B measures such as masks and vaccine passports. And that’s why I resigned,” he says.