Theresa May's Chequers plan for a Brexit deal would be "substantially worse than the status quo" for British businesses, Boris Johnson has warned.
The former foreign secretary used a surprise appearance at an economic report launch in Parliament to again attack the prime minister's negotiating strategy for leaving the EU.
He spoke at an Economists for Free Trade (EFT) event on Tuesday attended by a battalion of Tory Brexit big-hitters including Jacob-Rees Mogg, former Brexit secretary David Davis and his ex-deputy Steve Baker, former party leader Iain Duncan Smith and ex-Defra secretary Owen Paterson.
Mr Johnson sat front and centre and was applauded when his presence was noted by Mr Rees-Mogg.
Addressing EFT economist Patrick Minford at the launch of its world trade deal guide for Britain after Brexit, Mr Johnson said: "You made quite rightly the political objection that it (Chequers) turns us into a 'vassal state'.
"It seems to me there is also this particular economic objection that in abandoning our seat around the table in Brussels and continuing to accept the single market legislation ... 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."
He also argued that the UK should follow Donald Trump's example and slash taxes to create a "happy and dynamic economy".
Downing Street on Tuesday reiterated that Chequers 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".
The Prime Minister's official spokesman said Mrs May briefed the weekly meeting of the Cabinet on the upcoming informal EU summit in Salzburg.
"The PM said she expected EU leaders to take stock of the progress we have made in the negotiations on both the withdrawal and future relationship," the spokesman said.
"It will also be the first time leaders discuss together the UK government's white paper which put forward a series of credible and serious proposals.
"The PM said that building on the constructive engagement over the summer she would underline the importance to both parties of the UK remaining the EU's closest ally."
However, Mr Rees-Mogg said that Brexiteer Tory organisation ERG would be unveiling its own alternative plan for Northern Ireland on Wednesday.
The group has faced criticism from opponents who say it has attacked the Government's plans without coming up with one of its own on one of the main sticking point in discussions between Brussels and the Government.
In answer to a question, Mr Rees-Mogg said people could "come along and you will discover what our solution is to the Northern Ireland problem, which is the only thing that is obstructing" a free trade agreement dubbed "Canada-plus" with the EU.
He added: "It is possible to move very swiftly to a Canada-plus style deal as long as we can come up with a scheme, which I think we have got for tomorrow, on how do you ensure a solution to the Northern Ireland problem that any reasonable person would accept?"
