Long-serving MP and former chancellor Ken Clarke also said the prime minister for leading “government with no policies”.
Clarke, 76, is one of the most prominent pro-EU Tory politicians. He who called May a “bloody difficult woman” during the brief Tory leader contest, despite backing her.
“Nobody in the government has the first idea of what they’re going to do next on the Brexit front,” he told The New Statesman.
“Serious uncertainty in your trading and political relationships with the rest of the world is dangerous if you allow it to persist.”
He also said that, whatever deal Britain eventually struck to leave the EU, it would be denounced “by the ultra-Eurosceptics as a betrayal”.
May has been derided for not providing any detail of her plans for Brexit since entering Downing Street in July and mocked for simply saying: “Brexit means Brexit.”
Clarke said the prime minister faced “an appalling problem” of trying to get the “Three Brexiteers” -Foreign Secretary Boris Johnson, Brexit Secretary David Davis and International Trade Secretary Liam Fox - to agree with one another.
He said she would struggle to put together “a coherent policy which a united cabinet can present to a waiting Parliament and public. Because nobody has the foggiest notion of what they want us to do”.