Former UK Ambassador to the EU Ivan Rogers | Daniel Leal-Olivas/AFP via Getty Images UK’s ex-EU ambassador: Brexiteers like ‘snake oil salesmen’ Ivan Rogers accuses former Cabinet ministers of ‘breathtaking’ dishonesty.

The arguments of senior Brexiteers would "make snake oil salesmen blush" and their "dishonesty is breathtaking," according to the U.K.'s former ambassador to the EU.

Delivering a sobering lecture on the state of the Brexit process at University College London, Ivan Rogers, who resigned from the civil service in January 2017 blaming "muddled thinking" in government, warned that "denialism is pretty universal" on all sides of the Brexit debate.

"I believe the risks are appreciably higher than either the markets or the commentariat believe," he said, adding that "it is now genuinely urgent that we get beyond the myth-making."

Rogers' lecture contained criticisms for Theresa May's deal; Remainers seeking a "People's vote"; and Brussels negotiators alike. But his harshest words were reserved for the Brexiteers, who he said "avoid having to say anything serious or precise about what their destination means and how it works."

He said that former Brexiteer Cabinet ministers Boris Johnson, David Davis and Dominic Raab were dishonest for repeating the claim that the U.K. had been offered a Canada-style trade deal by European Council President Donald Tusk. Rogers said that in fact such a deal was only on offer to Great Britain, with Northern Ireland remaining in the EU's regulatory and customs regime — something that the Democratic Unionist Party, whose MPs support the government, would never agree to.

On the "canard" that the U.K. can simply leave the EU on World Trade Organization terms, Rogers called it "a gross dereliction of responsibility and a huge failure of leadership, under cover of increasingly empty demagogic rhetoric about betrayal."

Because a trade deal with the EU will ultimately be essential, he argued that "you obviously gain nothing by tumbling completely out to WTO rules, and then having to try and scramble your way back up the hill to a preferential deal, under huge time pressure."

"You just hand the perfect negotiating hand to the other side," he said.

And he predicted a future for the U.K. stuck in near permanent negotiation with the EU. "Life after Brexit will be bedevilled with one long, difficult negotiation after another with our nearest neighbors and biggest trading partnership. And those negotiations will force very hard choices on us."

"There is no leap to freedom which permanently ends this," he concluded.

Boris Johnson, David Davis and Dominic Raab did not respond to a request for comment by time of publication.

Charlie Cooper contributed reporting.