Speaking to on his podcast, the Streatham MP confirmed it was his decision to stand for the Liberal Democrats against Tory MP Mark Field in The Cities of London & Westminster if and when a national vote is called.
The seat, which is currently held by Mr Field with a 3,148 majority, is one of the most iconic in the country and encompasses 10 Downing Street, Buckingham Palace, the City and the West End.
Mr Umunna, 40, also denied he was making an "opportunist" move by leaving a constituency he has held since 2010 but always won on a Labour ticket.
The former Labour MP, who , told Acting Prime Minister: "I'm going to be standing in The Cities of London & Westminster. I've been appointed as the candidate there. This is a huge battle and highly symbolic.
"This part of central London ... not only is it a strongly Remain community. I know it well, I've worked here for the best part of a decade for businesses. Almost all my clients were in this constituency.
"I can see just how devastating leaving the European Union is going to be for so many of these businesses."
He added: "It really is taking it to Boris Johnson's doorstep. He currently lives in Downing Street and Number 10.
"On current polling it is an absolutely two-horse race between us and the Tories here and we're going to go for it."
Mr Umunna rejected the claim put to him by Brand that switching seats in London could be seen as opportunistic.
"Let's just look at the evidence right. Based on history you don't leave one of the two main parties if you are looking to be opportunistic," he said.
"I'm not coming into a Liberal Democrat safe seat. I'm coming into fight a seat where we are challenging the Tories.
"I'm not sure this 'opportunism' label based on the evidence is well made."
When it was suggested that he only switched because he wouldn't be able to win against Labour in Streatham, he said: "(I) completely disagree."
He added: "We've got a fantastic candidate ... The issue in Streatham is going to be who can you trust to promote the Remain cause. It scored the highest Remain vote in the country."
Acting Prime Minister has put guests ranging from Jacob Rees-Mogg and Jess Phillips to Ben Fogle and Bobby Norris in power and asked the question: "What would you do now?"
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