The former Ukip leader spoke at a packed Leave Means Leave rally in London, telling Brexiteers they must “face reality in the face”.
The MEP spoke alongside former Conservative leader Iain Duncan-Smith, MP Esther McVey and Hotelier Rocco Forte.
Mr Farage said he believed "it is now quite possible that we will see an extension of Article 50".
"When I've talked in the past about being worried that they may force us into a second referendum.
"I don't want it anymore than you do but I am saying to you we have to face reality in the face.
"Don't think the other side aren't organised, don't think the other side aren't prepared, don't think they haven't raised the money, don't think they haven't got the teams in place, they have."
The audience at the Leave Means Leave rally were fired up and heckling throughout the nights speeches.
Mr Duncan-Smith said Britain's "greatness" lies in the post-Brexit future.
He added: "I love this country dearly, I love it with all my heart.
"I love people whether they're Remainers or Leavers, I don't care.
"But I know one thing, this country's greatness lies ahead of it and we have an opportunity and a duty to deliver it.
"I pledge to you tonight, I will not sleep, I will not rest, I will not wake to find a Britain that is otherwise than independent and free once again."
He branded the EU a "political project that we have never fully been told the truth about" and described anti-Brexit arguments as "a load of rubbish".
Mr Duncan-Smith shared an anecdote from his time as work and pensions secretary during a visit to Berlin, when he said to German Chancellor, Angela Merkel, "just like the rest of you I've sat here for an hour and I've now lost the will to live".
But afterwards he was told his joke had translated in French to him being "terminally ill" rather than "losing the will to live".
And Ms McVey said she asked the cabinet to vote on Mrs May's deal before it was taken to the commons as she could not "put her name on it" and there were "dissenting voices" in the cabinet against it.
She said the decision was down to the Prime Minister whether to have a vote but she was "shouted down and told we were not going to do it".
Ms McVey said she and her fellow Brexit MPs "are fighting hard, fighting hard for you, and will not rest as a minority in parliament until you get what you voted for".