Addressing MPs - Mr Johnson said: "Given that in important ways this is Brexit in name only, I am of course unable to support it, as I said in cabinet at Chequers, and am happy to be able now to speak out against it".
Johnson, who led the main Brexit campaign in the 2016 referendum, resigned over May's strategy, triggering the government's biggest crisis since she lost her parliamentary majority after calling a snap election past year.
Twelve pro-EU Conservative MPs attempted to attach an amendment onto the government's trade bill last night which would have forced the government to join a customs union if it did not manage to secure a free-trade deal with the EU.
Tuesday's legislation was technical in nature - focusing on converting trade deals between the European Union and third countries into bilateral deals with Britain, and was not originally meant to define new trade policy.
At the time Johnson was delivering the speech, May was being subjected to a forensic cross-examination by the influential Liason Committee of MPs who grilled her about Brexit among other topics.
"We can't rely on anything", said Warren East, CEO of United Kingdom engine maker Rolls Royce, who said his firm could start stockpiling parts soon if it looked like Britain was heading towards a disorderly exit from the EU.
Chicago Residents Protest, Confront Police Hours After Fatal Shooting
Police rep Anthony Guglielmi says the victim was shot several times , but blames "inaccurate information" for fomenting the crowd. A string of police killings of black men, some unarmed, has caused outrage and led to protests in cities across the US.
May's plan has drawn criticism from members of her party as well as pro-Brexit members.
Guto Bebb says he's "distraught" to have had to quit as defence minister but says he couldn't vote for changes to key Brexit legislation after ministers "caved in" to pressure from Brexiteer MPs.
"We dithered and burned through our negotiating capital", he said.
In latest opinion polls UKIP has gained several points after its support collapsed in last June's snap general election which led to May losing her majority in the House of Commons.
"Let us again aim explicitly for that glorious vision of Lancaster House ... not the miserable permanent limbo of Chequers".
In his letter, Mr Davies continued: "Failure to keep our promise to the electorate will nearly certainly lead to the catastrophe of Jeremy Corbyn becoming Prime Minister and I can not sit back and allow that to happen". The BMA has consistently highlighted the importance of a formal agreement post Brexit, and we want to see the government commit to getting the best deal possible for patients and doctors on this issue.
Explaining that the two states already have the same rules and laws, the Tory MP told the BBC: "The only reason we wouldn't come to a free and open agreement is because politics gets in the way of economics".