The Scot let his allegiances be known as he led a discussion with Chancellor George Osborne at a conference in London, when asked about the upcoming referendum in Scotland on 18 September.
When the chancellor said "the more the people of Scotland focus on the benefits of the UK they will be clear they want to stay in the UK", Flint replied: "Absolutely agree."
The HSBC chairman is backing the Better Together campaign being led by former chancellor Alistair Darling. Flint declined to elaborate on his comment, while HSBC said he was speaking in a personal capacity. HSBC does not have a large presence in Scotland, with just 10 branches.
Darling, who was also attending the conference, told Reuters that the vote was "going to be closer than people think" even though polls suggest Scots will vote to stay in the union.
Darling also spoke about Europe at the Institute of International Finance conference in London, where he argued that the argument about staying in Europe needs to made as if the UK were on the outside. Amid a row over whether Luxembourg's Jean-Claude Juncker will become the next European commission president, MEP Sharon Bowles said: "He is a man who understands markets."
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