But why did Lord Frost resign and what did he say?
Here’s what you need to know.
Lord Frost is reported to have handed in his resignation letter to the Prime Minister last week, with an agreement to leave in January 2022.
However, in a letter to Mr Johnson released on Saturday evening (18 December), he said that he was “disappointed that this plan has become public” and “in the circumstances I think it is right for me to write to step down with immediate effect”.
Lord Frost - who has led negotiations with the EU - thanked Mr Johnson and said “Brexit is now secure”.
He added: “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.”
Lord Frost also said he was sad the unlocking from Covid-19 restrictions had not proved “irreversible” as promised, and added: “I hope we can get back on track soon and not be tempted by the kind of coercive measures we have seen elsewhere.”
He also said he hoped that the UK would become a “lightly regulated, low-tax” country.
In response to Lord Frost’s announcement, Mr Johnson said he was “very sorry” to have received his resignation.
Resignation letter in full
Lord Frost’s full resignation letter read:
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 onto 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.