Speaking at the party leadership hustings in Cardiff on Sunday, Ms Thornberry called Mr Johnson a liar for saying he will agree a vital trade deal with the EU by the end of the transition period on December 31.
It contradicts ex-European Council president Donald Tusk’s earlier claim that “one year is enough” to get an agreement.
In a theatrical response to a question about post-Brexit workers’ rights, the Islington South and Finsbury MP told the hustings: “We’ve technically and constitutionally and legally left the European Union. Boris Johnson has done what he promised: he was going to get Brexit done. But he lied!
“He’s not going to have got Brexit done because we need to continue to have a relationship with the European Union. Half of our trade is with the European Union, we need to get a deal in the next year.
“He says he can get it done in a year and he lies. Because what’s he done in the last month? He’s decided to have a big battle over whether or not Big Ben should bong. He’s decided to produce a 50p coin. That’s what he has done, he is lying.”
Ms Thornberry went on: “He’s not going to get Brexit done by the end of the year and so what will he do? When he can’t get Brexit done, he can’t get the deals and he decides he has to paddle over the Atlantic to go into the small, horrible, wet, damp hands of Donald Trump, what’s he going to do?
“He’s going to undermine workers’ rights, he’s going to try and make us some offshore island to the rest of the European Union with lower standards, where it will be cheaper and easier to be able to exploit people. And we say ‘no’, we say: ‘No, we are not going to have this. You do not undermine our country.’”
She signed off: “We will not allow our communities to be undermined by Boris Johnson and we have to fight back together.”
Read more from Yahoo News UK
The shadow foreign secretary’s attack on Mr Johnson extends from her key leadership campaign narrative of holding the PM to account in the House of Commons, having previously clashed with Mr Johnson over the despatch box when he was foreign secretary.
According to the Oddschecker website, Ms Thornberry remains the rank outsider in the leadership race, with some bookmakers offering odds as long as 100/1. The other contenders are Keir Starmer, Rebecca Long-Bailey and Lisa Nandy.