The then home secretary told an audience of investment bankers that she had serious concerns about the effects of leaving the EU, according to a leaked recording of her meeting to 'The Guardian'.
She said: "I think the economic arguments are clear. 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.
She also said that Britain's security was best served inside the EU.
"There are definitely things we can do as members of the European Union that I think keep us more safe," she said.
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The leaked remarks appear to go against the general public impression that she was a reluctant supporter of the Remain camp, led by the then Prime Minister David Cameron.
"David Davis (Brexit minister) made very clear in the House of Commons last week the importance the government places on financial services across the UK in the negotiation to come, as has the chancellor in recent weeks.