Speaking at the Ukip party conference in Bournemouth today, Farage said that without strong opposition from the Labour party "the temptation of the Prime Minister will be to go for a soft Brexit".
Leadership update: New Ukip leader confirmed
He said May intended to take back "some control" of the UK's borders, but not total control, and claimed the home secretary Amber Rudd is "still fighting the referendum".
"No, Prime Minister, we voted to take back control of our borders, simple as that," he said.
As soon as he stepped onto the stage at the Ukip conference, Farage hailed the outcome of the EU referendum, saying: "We did it, we got our country back."
"It felt like a fairy tale that had come true," he said. "And we've brought down the Prime Minister, and we've got rid of a chancellor."
Ukip supporters applauded loudly as Farage approached the stage. One shouted: "Thank you, thank you, you bring me hope."
Farage resigned as Ukip leader in July after he achieved his political ambition to take Britain out of the European Union. The candidates to succeed him include MEPs Diane James and Bill Etheridge, councillor Lisa Duff and Liz Jones and Philip Broughton.
A result is expected at 13:35 today.
On City A.M. today: David Davis MP on Britain's bright post-Brexit future
It's been a tough race to the finish line; two of the would-be leaders were knocked out off the running in August.
Steven Woolfe, once the favourite to win, missed the deadline to enter his papers online and Jonathan Arnott withdrew later in the month because he wasn't convinced he could win.
Farage has not publicly endorsed any of the candidates and has said he will now focus on his job as an MEP to ensure Britain gets the best possible deal in its negotiations with the EU.