A leaked recording of a speech by Theresa May sheds some light on the depth of the prime minister's opposition to taking Britain out of the European Union before the referendum earlier this year.

May was Home Secretary at the time of the speech to investment bankers at Goldman Sachs. It was delivered in May, less than one month before the historic Brexit referendum vote in favor of leaving the EU.

She had publicly opposed Brexit but was criticized for not being more vocal about her position.

In the recording, published Wednesday by the Guardian, May told bankers she believed Britain reaped "significant" benefits from being part of the European single market, with its 500 million people.

She warned firms might leave Britain if it was no longer part of the 28-nation bloc. She later changed her view.