A high-profile interviewer and columnist was at the centre of a plagiarism row and a Twitter storm on Tuesday after he said he had added quotes into interviews he had conducted that were taken from the subject's writings and not from the interview itself.

Johann Hari, who has written for the Independent over the past decade, said in a blogpost entitled "Interview etiquette", written late on Monday, that when "I've interviewed a writer" he had "occasionally" chosen to quote "the idea as they expressed it in writing, rather than how they expressed it in speech" to make their thoughts clearer.

The Orwell prize-winning writer was responding to a number of bloggers who had accused him of plagiarism in the past few days, starting with the so-called Deterritorial Support Grouppppp (DSG), who compared almost identical quotes in a 2004 interview conducted by Hari with the Italian Marxist Antonio Negri with a 2003 book, Negri on Negri, by Anne Dufourmantelle. Other bloggers found similar examples from as recently as last year.

Hari wrote that he was "a bit bemused to find one blogger considers this 'plagiarism'" and described most of his interviews as "intellectual portraits".

He said that "after doing what must be over 50 interviews, none of my interviewees have ever said they had been misquoted" and added that he had called around other unnamed newspaper interviewers "and they said what I did was normal practice".

His remarks, though, prompted a flurry of mainly hostile tweets, and the hashtag #interviewsbyhari ranked third in the trending categories on Twitter worldwide by Tuesday lunchtime. However, as the row developed, several high-profile journalists came to his support, including Guardian columnist George Monbiot and Caitlin Moran from the Times, who tweeted: "Johann Hari generally writes good, thoughtful, brave journalism, and makes Fleet Street a better place."

As the row developed, Hari remained silent, saving his reaction for a comment piece published in Wednesday's Independent and the i.

Hari used the article to concede that, on reflection, the approach he had sometimes taken was wrong, but denied accusations of plagiarism and churnalism. "Every word I have quoted has been said by my interviewee, and accurately represents their view.

"It's clearly not plagiarism or churnalism – but was it an error in another way? Yes. I now see it was wrong, and I wouldn't do it again. I'm grateful to the people who pointed out this error of judgment."

However, the allegations earlier prompted the organisers of the Orwell prize, which bills itself as "Britain's most prestigious prize for political writing" and was won by Hari in 2008, to consult the committee that made the award to see if it should take further action.

Entitled Hari Kari/Hackery, the original DSG posting, written on 17 June, cited a quote Negri gave Hari on the subject of memory in the 2004 interview. "Who controls memory? Faced with the weight of memory, one must be unreasonable! Reason amounts to eternal Cartesianism. The most beautiful thing is to think 'against', to think 'new'. Memory prevents revolt, rejection, invention, revolution."

The DSG blog (the writer has not chosen to reveal his or her identity) goes on to cite a passage from pages 100 and 101 of the Dufourmantelle book written a year earlier. That passage uses almost identical language to the interview quotes, beginning: "Who controls memory? Faced with the weight of memory, one must be unreasonable! Reason amounts to eternal Cartesianism."

The book then has half an extra sentence – "As against Descartes, one ought to choose Galileo instead" – before continuing with language almost identical to that which appeared in the 2004 interview by Hari. "The most beautiful thing is to think 'against', to think 'new'. Often, memory prevents revolt, rejection, invention, revolution."

The blog goes on to cite several other examples of similar language in the book and interview.

That was followed by Brian Whelan, editor of Yahoo! Ireland and blogger, who examined other Hari interviews, doing what he called "a basic check for plagiarism". He compared a September 2010 interview with controversial Israeli journalist Gideon Levy with writings by Levy in Haaretz, the newspaper for which he works.

Whelan found several passages in the interview that appeared to be composed of sentences that had appeared in columns previously written by Levy – and by other journalists describing him.

Hari's interview read: "With a shake of the head, he says: 'We had now two wars, the flotilla – it doesn't seem that Israel has learned any lesson, and it doesn't seem that Israel is paying any price. The Israelis don't pay any price for the injustice of the occupation, so the occupation will never end. It will not end a moment before Israelis understand the connection between the occupation and the price they will be forced to pay. They will never shake it off on their own initiative.'"

In July 2007, Levy wrote something very similar in a column for Haaretz: "The Israelis don't pay any price for the injustice of the occupation, so the occupation will never end. It will not end a moment before the Israelis understand the connection between the occupation and the price they will be forced to pay. They will never shake it off on their own initiative, and why should they?"

However, when contacted by the Guardian, Levy said he was not unhappy: "I stand behind everything that was published in the interview, which was an accurate representation of my thoughts and words."

As the online row began to subside, Simon Kelner, the Independent's editor in chief, issued a moderately supportive statement, but did not say whether any of Hari's interviews with the inserted quotes would be reviewed or whether the journalist would be disciplined. Kelner said: "Johann Hari has worked for the Independent for more than 10 years, winning a number of international journalism awards. He explained his position in a personal blog, which he threw open to debate. He has listened and reflected on the range of views expressed and will be writing about it in the Independent and i."

Hari's comment piece is to be published in the Independent – and will be available online – on Wednesday.

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