"Making it to the shortlist on a first novel is sort of like winning and anything beyond that is quite a bonus," Adiga said.
He said the book was set in today's India and "revolves around the great divide between those Indians who have made it and those who have not".
"At the heart of the book it is something existential," he added. "It's a quest to break out of the circumstances you find yourself in - it's a quest for freedom."
Announcing the winner at a ceremony in London, Mr Portillo said: "My criteria were 'Does it knock my socks off?' and this one did ... the others impressed me ... this one knocked my socks off."
Aravind Adiga on winning the Man Booker prize for The White Tiger
Mr Portillo said what set the book apart was its originality in showing "the dark side of India".
He said: "The novel is in many ways perfect. It is quite difficult to find any structural flaws with it."
There were more than two contenders for the prize on the shortlist, but the winner was "absolutely not a compromise", Mr Portillo added.
"There really was a decision. The judges were asked to express their satisfaction and they all did."
Adiga is the fourth first-time novelist to win the prize. Previous debut winners were Keri Hulme's The Bone People in 1985; Arundhati Roy in 1997 for God of Small Things; and Vernon God Little by DBC Pierre in 2003.
Adiga is a former correspondent for Time magazine and has written for the Independent and the Sunday Times.
The win means he can expect an upturn in sales and added recognition.
According to Amazon.co.uk, the six books enjoyed average sales rises of 700% following the announcement of the shortlist last month.
Booksellers Foyles said it was a "very exciting winner for bookshops" adding it could prove as popular as The Life of Pi, the Bookers best-selling winner.
The five other shortlisted authors can also expect a rise in sales
Adiga, who had been given odds of 7/1 to win before the ceremony, dedicated the award to "the people of New Delhi".
Irish writer Barry had been tipped to take the prize at 7/4 by bookmakers William Hill.
The bookmakers' favourite has not won since Yann Martel in 2002.
Of the six authors in contention for this year's Man Booker, only Barry had ever been shortlisted in the past - in 2005 for A Long Long Way.
Grant and Hensher had both previously been longlisted.
The award, which honours the best fiction written in English by an author from the UK, Ireland or the Commonwealth, was handed out at the Guildhall in London on Tuesday.
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