The recording seems to show her expressing numerous concerns about leaving the EU– which is in sharp contrast to her public speeches in support of Brexit.
In the speech, which was made on 26 May, the now-PM says: "I think the economic arguments are clear.
"I think being part of a 500-million 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."
Since the speech emerged, there have been many MPs speaking out, including Andrew Gwynne, Labour MP for Denton, who told the Evening Standard: “As if we needed it, this recording is cast-iron evidence of how Theresa May and other senior Tories have been saying one thing in private about the economic impact of Brexit, and another in the comfort of Tory conference halls.”
Liberal Democrat leader, Tim Farron also weighed in: "It is disappointing that Theresa May lacked the political courage to warn the British public as she did a group of bankers in private about the devastating economic effects of Brexit.
"But far more disappointing is that now she is supposedly in charge, she is blithely ignoring her own dire warnings.”
A government spokesperson released a statement on the leaked recording, saying: “Britain made a clear choice to vote to leave the EU and this Government is determined to make a success of the fresh opportunities it presents.
"We want a smooth and orderly exit from the European Union, which would be in the interests of both Britain and the EU."