An unidentified intruder managed to bluff her way in to the Republican retreat where Theresa May spoke, and possibly covertly recorded audio of private meetings.
President Donald Trump and Vice President Mike Pence travelled with the Prime Minister to the closed door gathering in Philadelphia last week.
Investigators are now examining how the woman, who posed as the wife of an unnamed Republican politician, was able to gain access.
It is believed she passed through the rigorous security procedures and possibly used fake credentials, raising security concerns.
The breach was announced by Mark Strand, head of the non-profit Congressional Institute which organised the retreat.
In an email to attendees obtained by The Associated Press, Mr Strand said the woman “misrepresented herself on multiple occasions to retreat organisers as the spouse of an elected official”.
He added: "The Congressional Institute is continuing to investigate this breach in order to fully understand how it happened and to ensure it does not happen again."
The revelation came as organisers scrambled to find out how audio from inside the retreat was leaked to various news outlets.
On the audio, GOP politicians can heard airing concerns about the practical difficulties and political risks of repealing and replacing President Barack Obama's health care law, also known as Obamacare.
A question-and-answer session with Mr Pence was also recorded. It was not immediately clear if the intruder was present for the session with Mr Trump, who spoke before the Vice President but did not engage in a private discussion with politicians.
Chair of the conference, Representative Cathy McMorris Rodgers, said: “Leaks from internal member discussions are unacceptable."
Additional reporting by AP
