His decision to walk followed that of Brexit minister David Davis, leading to open speculation about whether Prime Minister Theresa May could face a leadership challenge.
Mr Johnson wrote a lengthy letter explaining why he felt he had to go. We've published it in full below.
It is more than two years since the British people voted to leave the European Union on an unambiguous and categorical promise that if they did so they would be taking back control of their democracy.
They were told that they would be able to manage their own immigration policy, repatriate the sums of UK cash currently spent by the EU, and, above all, that they would be able to pass laws independently and in the interests of the people of this country.
Brexit should be about opportunity and hope. It should be a chance to do things differently, to be more nimble and dynamic, and to maximise the particular advantages of the UK as an open, outward-looking global economy.
That dream is dying, suffocated by needless self-doubt.
We have postponed crucial decisions — including the preparations for no deal, as I argued in my letter to you of last November — with the result that we appear to be heading for a semi-Brexit, with large parts of the economy still locked in the EU system, but with no UK control over that system.
It now seems that the opening bid of our negotiations involves accepting that we are not actually going to be able to make our own laws. Indeed we seem to have gone backwards since the last Chequers meeting in February, when I described my frustrations, as Mayor of London, in trying to protect cyclists from juggernauts. We had wanted to lower the cabin windows to improve visibility; and even though such designs were already on the market, and even though there had been a horrific spate of deaths, mainly female cyclists, we were told that we had to wait for the EU to legislate on the matter.
So, at the previous Chequers session we thrashed out an elaborate procedure for divergence from EU rules. But even now that seems to have been taken off the table and there is in fact no easy UK right of initiative. Yet if Brexit is to mean anything, it must surely give ministers and Parliament the chance to do things differently to protect the public. If a country cannot pass a law to save the lives of female cyclists — when that proposal is supported at every level of UK government — then I don't see how that country can truly be called independent.
Conversely, the British Government has spent decades arguing against this or that EU directive, on the grounds that it was too burdensome or ill-thought out. We are now in the ludicrous position of asserting that we must accept huge amounts of precisely such EU law, without changing it an iota, because it is essential for our economic health — and when we no longer have any ability to influence these laws as they are made.
