Mr Johnson also indicated his readiness to join a united Eurosceptic front against Downing Street by lining up with other senior Tories to back a report highlighting the potential benefits for Britain of quitting the EU without a deal.
The politician made a surprise appearance at Westminster for the launch of a pamphlet calling for a “world trade deal” published by the Economists For Free Trade group.
Other leading Tory Eurosceptics, including MP Jacob Rees-Mogg and former EU exit secretary David Davis, were also at the launch held in a Commons committee room.
The report argues that the UK could enjoy a sustained economic boost by adopting a relationship with the EU based on the existing World Trade Organisation rules rather than negotiating a new trade deal.
Mr Johnson said Economists For Free Trade were right to warn that Mrs May’s Brexit plan, agreed by the Cabinet at her Chequers country retreat in July, would turn the UK into a “vassal state” forced to accept EU regulations without a say.
He said: “We will be exposing UK businesses, manufacturers, entrepreneurs, innovators to whatsoever rules the EU decides in the future to devise, even though those rules may well be inimical to the interests of UK innovation.
“That seems to me to be a particular economic risk in ‘Chequers’ and makes it substantially worse than the status quo.”
His blast yesterday followed a series of scathing attacks on the Prime Minister’s plans in recent days, including his likening her Chequers proposals to putting a “suicide vest” around the British constitution.
Mr Rees-Mogg told the meeting: “We want to achieve a deal, but what is the best way to achieve a deal?
“Is it to be snivelling and fearful and give the impression to the EU that we will do whatever they say, that we will go down on bended knee and offer to serve homage to the great power that there is within the EU?” Instead, Britain should show that “we have got nothing to be fearful about”, he said.
The Economists For Free Trade document argues that trade based on World Trade Organisation rules could deliver an £80billion boost to Treasury coffers from increased growth.
Last night, the Prime Minister’s spokesman said the Chequers proposal was “the only serious, credible and negotiable plan which is on the table which both delivers on the will of the British people and which prevents the imposition of a hard border in Northern Ireland”.
Proposals for breaking the Brexit negotiation deadlock over the Irish border will be unveiled today by leading Tory Eurosceptics.
Mr Rees-Mogg’s European Research Group is expected to unveil a detailed plan showing how the UK and the EU can avoid the need for new border infrastructure, including strict customs checks for crossings between Northern Ireland and the Irish Republic after Brexit.