The former Ukip leader said Mrs May’s speech showed the prime minister had “laudable intentions”, but left him wondering if she had the authority, power and strength to ensure Brexit happens.
During her speech in Mansion House, Mrs May spoke about regaining control of Britain’s laws, borders and money and recognised that the referendum “was not a vote for a distant relationship with our neighbours”.
But Nigel Farage wrote: “Where is her authority, her power, her strength and her influence? For without these things, this country is not in charge of its destiny.
“What I find myself wondering more and more, though, is whether the prime minister has the sheer force of personality required to make sure these aims are met.”
He said Theresa May had too many other “preoocupations in her political life” including Brussels bullies, Remainers like Anna Soubry and a “wafer thin majority” to focus on Britain’s exit.
Writing for the Daily Telegraph, Mr Farage said: “I am the first to admit that even with a fair wind behind even the most successful prime minister, securing a successful Brexit while making sure the country was not badly split by it would be a big task.
“The trouble for Mrs May is that that fair wind isn’t blowing for her. There are too many other preoccupations in her political life.
“But on top of this she has to try to take on the Brussels bullies, that intractable group who seem hell bent on making the withdrawal process as complicated for Britain as possible.”
Mrs May compaigned to remain in the EU before she was a prime minister.
Mr Farage accused the PM of not doing enough to convince members of the public she believed in Brexit.
He said: “Mrs May has failed fully to quell suspicions she is not up to that job. It was telling that when asked the question ‘Is Brexit worth it?’ after her speech, she couldn’t bring herself to answer.
“This on top of her being unable to say how she would vote if there was another referendum held today.
“She still has time to show her mettle, but the clock is ticking. She must act now or risk damaging this great nation of we she spoke so proudly yesterday. And she must do so in a way that will guarantee her name will appear in the history books for all the right reasons.
Mrs May on Friday vowed to secure best trade deal in the world with the EU after Brexit while promising to keep the UK united with no Irish border and and setting out five tests that must be met for a deal with the bloc.
Mrs May said she wants Britain to have the freedom to strike trade deals around the world, but also agree a Brexit relationship with the EU where they continue to “support each other’s interests”.
And she said five tests must be met to guide Britain’s approach to the negotiations.
She stressed the agreement should strengthen “our union of nations and our union of people”, in a clear allusion to Northern ireland.