In an audio recording leaked to The Guardian, Ms May privately told Goldman Sachs she had concerns about the security and financial status of the UK if the nation were to vote to leave the EU.
During the campaign the then-Home Secretary said she backed Remain, but was publically muted in her support, creating tensions with David Cameron. One of the ex-Prime Minister's aides has since suggested her reserved position helped her career in the aftermath of the Leave vote.
The leaked tape comes after she indicated in her first conference speech as Prime Minister that the UK would embark on a so-called “hard Brexit”, leaving the single market and ending freedom of movement. The suggestion further shook the already-fragile value of the pound.
One month before the referendum, Ms May told a secret audience at the bank it was time the UK was more active within the EU. She expressed fears that businesses would leave the country if voters chose to exit the union.
“I think the economic arguments are clear,” she said in audio obtained by The Guardian. “I think being part of a 500-million population trading bloc is significant for us. I think, as I was saying to you a little earlier, that one of the issues is that a lot of people will invest here in the UK because it is the UK in Europe.
“If we were not in Europe, I think there would be firms and companies who would be looking to say, do they need to develop a mainland Europe presence rather than a UK presence? So I think there are definite benefits for us in economic terms."
She also said the UK's security situation was better as part of the EU, where intelligence was shared among constituent countries.
"There are definitely things we can do as members of the European Union that I think keep us more safe," she added.
During the Conservative Party conference she said she would not commit the UK to the single market during Brexit negotiations, and would instead focus on controlling immigration.
The audio leak follows a damning memoir by David Cameron's former director of communications, which claimed Ms May rebuffed Mr Cameron on more than a dozen occasions when he implored her to back the Remain campaign.
The book said No 10 dubbed the then Home Secretary "Submarine May", because she always disappeared when she was needed.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies