It on top of another very hard day for the prime minister, after narrowly surviving a key vote on the customs union last night, and caving to the demands of hardline Tory Brexiteers in a series of votes on Monday.
Elsewhere, UK Prime Minister Theresa May was visiting Northern Ireland and the Irish border on Thursday, ahead of a speech tomorrow in Belfast where she will emphasise the government's commitment to avoiding a hard border on the island of Ireland and protecting the Good Friday agreement.
The International Monetary Fund said on Thursday that European Union countries will suffer long-term damage equivalent to about 1.5 percent of annual economic output if Britain leaves the bloc without a free trade deal next year.
Britain and the 27 European Union nations both say they want a smooth Brexit when it officially takes place in March, but talks over the past year have got bogged down amid deep political divisions in London on what strategy to take.
May is battling to unite her Conservative Party around her blueprint, formally unveiled last week following months of cabinet infighting.
Turmoil over Brexit plans has hit the pound.. The document proposes keeping Britain and the European Union in a free market for goods, with a more distant relationship for services.
The intervention was cheered by pro-Brexit members of parliament but after he failed to detail how he would achieve a Brexit that does not ravage the economy, one sceptical lawmaker cried out: "Is that it?"
"What should have become clear to [May] is that the Chequers proposals are completely untenable with our [party] membership in the country and the electorate", Andrew Bridgen, a Conservative pro-Brexit lawmaker, told Reuters.
Johnson said that would deliver "Brexit in name only" and leave Britain in a state of "economic vassalage".
May said Wednesday that talks had already started with Brussels based on the proposal set down in a white paper policy document earlier this month.
The call for preparation comes against the backdrop of the UK's internal political strife, with British prime minister Theresa May seeing two of her most senior cabinet members resign last week because of disagreements over the Brexit deal, delaying progress on talks.
Conservative lawmakers fear an election, and the possible victory of veteran socialist Jeremy Corbyn.
"Michel has told us the clock is ticking".
Johnson said the Lancaster House approach had been welcomed by commentators and the financial markets, but that "we never actually turned that position into a negotiation. instead we dithered, and we burned through our negotiating capital".
Johnson, who led the main Brexit campaign in the 2016 referendum, resigned this month over May's strategy, triggering the government's biggest crisis since she lost her parliamentary majority after calling a snap election a year ago.
After Mr Johnson used his resignation statement in the Commons to call for the PM to abandon her "miserable" version of Brexit, Mrs May's former joint chief of staff Nick Timothy said the current situation was "deeply depressing".
Mr Clarke said he had withdrawn a letter calling for a confidence vote in Mrs May because Brexit had taken the Tory party to "breaking point", stating: "We have looked into the abyss in the last few days".
"We would need to consider what action we would take in those circumstances", she added.