Mr Johnson appeared to break with government policy by saying the UK should not pay for access to the European single market during a transition period, and returned to the controversial figure on the UK’s contributions to the EU budget which has been widely condemned as false.
Speaking as the Lib Dem conference gets underway in Bournemouth, Mr Cable called the Foreign Secretary a “Poundland Donald Trump” while the SNP challenged Mrs May to make clear who is setting Brexit policy.
Mr Johnson was criticised for the timing of his intervention, which came less than 24 hours after a failed terrorist attack in London that injured dozens of people, and less than a week before the Prime Minister is expected to set out proposals for a Brexit transition in a major speech in Florence.
He later tried to dampen speculation that he was laying the groundwork for another leadership bid, by posting a link to his article on Twitter with the message: “Looking forward to PM’s Florence speech. All behind Theresa for a glorious Brexit”.
In his 4,000 word article for the Daily Telegraph, the Foreign Secretary said continued membership the single market and customs union would make a “complete mockery” of the EU referendum result.
He said Brexit would make the UK “the greatest country on earth,” adding: “This country will succeed in our new national enterprise, and will succeed mightily.”
Reprising the most controversial promise of the Leave campaign in last year’s referendum, Mr Johnson wrote: “Once we have settled our accounts, we will take back control of roughly £350 million per week. It would be a fine thing as many of us have pointed out if a lot of that money went on the NHS, provided we use that cash injection to modernise and make the most of new technology.”
Making no mention of a transition period, Mr Johnson went on: “We would not expect to pay for access to their markets any more than they would expect to pay for access to ours”.
And he argued for post-Brexit tax reform, once the UK regains power over VAT rates.
Mr Johnson has also been criticised for questioning the loyalties of pro-EU young people, saying about protesters wearing the EU flag: “I look at so many young people with the 12 stars lipsticked to their faces, and I am troubled with the thought that people are beginning to have genuinely split allegiances.”
Mr Cable said: “Boris Johnson is on manoeuvres, which means everyone else should take cover.
“He clearly thinks that Theresa May is on the verge of a U-turn which would lead to a
transitional deal that would keep us in the single market.
“Theresa May must slap down Boris Johnson in the strongest terms or she will lose the last vestige of her authority to negotiate Brexit.
“Britain desperately needs political grown-ups who will put the country first, not a Poundland Donald Trump like Boris Johnson.”
SNP Europe spokesman Stephen Gethins said: “Theresa May must make clear whether Boris Johnson is speaking for the UK government with his latest embarrassing intervention on Brexit.
“The Tory government’s plans for Brexit already threaten significant damage to the UK and Scottish economies, and to our international reputation - Boris seems determined to make that even worse.”