Prime Minister Boris Johnson has suffered a fresh blow to his wounded leadership after his Brexit minister quit over disagreements about the UK’s Covid-19 and economic strategies. David Frost — one of the prime minister’s key allies who was one of the architects of the country’s exit from the European Union — quit saying he was concerned about the government’s “current direction of travel”. Senior Conservative MPs warned that Mr Johnson needs to get a grip on his party after the loss of a minister who had been at the centre of government decision-making. Mr Frost's resignation follows the biggest rebellion yet against Mr Johnson, as more than 100 MPs voted against his plans for tighter restrictions to combat the rapid spread of the Omicron variant of Covid-19. And amid continuing anger over continuing revelations about government Christmas parties in 2020 while the country was in lockdown, his party lost a 23,000 majority in a humiliating by-election defeat. In his resignation letter, Mr Frost urged the prime minister not to be tempted by the “kind of coercive [Covid-19] measures we have seen elsewhere”. The departure of Mr Frost will be seen as a major blow for the prime minister — not just because he led the UK’s Brexit project that will frame Mr Johnson’s legacy but because he remained a key part of his administration. Tobias Ellwood, a senior MP from Mr Johnson’s party, told Times Radio that it was worrying that somebody at the heart of the party had decided that “this isn’t the party I want to be part of”. Health Secretary Sajid Javid said that he understood the reasons why Mr Frost had quit. “Principled people do resign from the government — I know all about that — and that's something … that's a decision for him to take,” he told Sky News. MPs have said that Mr Johnson needs to reassert his leadership of the party if he wants to lead it to the next election in 2024. The by-election in North Shropshire which resulted in a damaging loss for the government was only held because one of Mr Johnson's MPs quit over a lobbying scandal, and a bungled attempt to change the rules on how politicians’ conduct was scrutinised. Then, Cabinet Secretary Simon Case — the country's top civil servant — recused himself from an investigation into whether Christmas parties held last year in government offices broke Covid lockdown rules after reports that he also attended a party. In his parting shot, Mr Frost expressed his wish that the UK would become a “lightly regulated, low-tax” country. Prominent Conservative MP Andrew Bridgen said Mr Johnson was “running out of time and out of friends to deliver on the promises and discipline of a true Conservative government". “Lord Frost has made it clear, 100 Conservative backbenchers have made it clear, but most importantly so did the people of North Shropshire,” he wrote on Twitter. The opposition Labour Party’s deputy leader, Angela Rayner, said the news showed “a government in total chaos right when the country faces an uncertain few weeks". She tweeted: "@BorisJohnson isn't up to the job. We deserve better than this buffoonery.” Mr Frost has recently been locked in talks with European Commission Vice-President Maros Sefcovic as the UK and the EU attempt to close gaps in post-Brexit arrangements. He wrote in the <i>Financial Times</i> on Friday warning the EU it needed to compromise but without straying into any of the areas where he clashed with government. Dear Boris, I have led our EU exit process for the two and half years since you became prime minister. In those years we have restored the UK's freedom and independence as a country and begun the process of building a new relationship with the EU. That will be a long-term task. That is why we agreed earlier this month that I would move on in January and hand over the baton to others to manage our future relationship with the EU. It is disappointing that this plan has become public this evening and in the circumstances I think it is right for me to write to step down with immediate effect. It has been a huge honour and privilege to work with you over the last five years, first in the Foreign Office and then in No. 10. You have been an outstanding leader at a moment of grave constitutional crisis for this country. Many said that it would be impossible to deliver what we did: an end to political turbulence by implementing the referendum result, a stunning election victory, an exit from the EU which gave us full freedom about our future choices as a country, and finally putting in place the world's broadest and indeed only zero-tariff free trade deal. You and I have always shared the same approach on Brexit and I do not think we would have achieved so much without that close common understanding of our aims. Brexit is now secure. The challenge for the Government now is to deliver on the opportunities it gives us. You know my concerns about the current direction of travel. I hope we will move as fast as possible to where we need to get to: a lightly regulated, low-tax, entrepreneurial economy, at the cutting edge of modern science and economic change. Three hundred years of history show that countries which take that route grow and prosper, and I am confident we will too. We also need to learn to live with Covid and I know that is your instinct too. You took a brave decision in July, against considerable opposition, to open up the country again. Sadly it did not prove to be irreversible, as I wished, and believe you did too. I hope we can get back on track soon and not be tempted by the kind of coercive measures we have seen elsewhere. Together we have put this country on to a new path. I am confident that under your leadership this newly free Britain can succeed and prosper hugely. I wish you and the Government every success in that. David