Will Lewis, the former Daily Telegraph editor, has refused to tell the Leveson inquiry whether he knew anything about how the BBC's Robert Peston obtained a leak of a covert interview with Vince Cable from the title's newsroom.

The newspaper journalist – who had by the time the Cable story was published in December 2010 joined the management team of News International – was asked on Tuesday by Robert Jay QC, counsel for the inquiry, whether he was the source of the leak from his former employer.

Replying, Lewis said: "I can't assist you with that, as you know, core to any journalist is the protection of journalistic sources, whether they are my source or someone else's source."

He continued: "Any way that I answer that question, as helpful as I would like to be, would endanger that principle" and concluded that it was not a matter he felt able to assist the inquiry on at all.

In December 2010 the Daily Telegraph sent two journalists to pose as constituents of Cable, the business secretary, and secretly recorded the Liberal Democrat MP criticising Rupert Murdoch at a time when he was adjudicating on the media mogul's proposed buyout of BSkyB.

That recording was not first published by the Telegraph, but instead leaked from the newspaper and was broadcast by Peston, the BBC business editor. Peston said that an unnamed "whistleblower" had passed him the tape of the interview.

Subsequently the Telegraph Media Group, the owner of the newspaper, hired Kroll Associates, the private investigators, which concluded that it had a "strong suspicion" that Lewis had orchestrated the leak. Lewis had left the Telegraph in May 2010, and joined News International as group general manager later that year.

When Jay first raised the subject of the Kroll investigation, Lewis told the Leveson inquiry that he had "left the Telegraph in May 2010 so I've no idea if the Telegraph conducted such an investigation". Later, as Jay tried to probe Lewis further on the subject, a security alarm went off – prompting laughter in the courtroom.

Peston's disclosure about Cable's "war on Murdoch" remark had a significant impact at the time. Cable was stripped of his responsibility for deciding on News Corp's bid for BSkyB, and it was widely believed that his successor in the task, Jeremy Hunt, would have been more sympathetic to the bid.

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