Sunday Times journalist Mazher Mahmood, a former reporter at the News of the World who became known as the 'fake sheikh', told the Leveson inquiry today that allegations he tried to bribe a member of staff at the Sunday Times in 1988 to alter news agency are "completely untrue".

At the beginning of his second appearance before the inquiry, Mahmood was asked about his departure from the Sunday Times in 1988.

In December last year Guardian media commentator Roy Greenslade claimed that Mahmood had "falsely blamed the news agency" after making a mistake in an article.

Greenslade also said in his December article that Mahmood "tried to back up his version of events by entering the room containing the main frame computer in order to alter the original copy".

In his evidence to the inquiry today, also recorded by the Guardian, Mahmood said he "made a mistake" and that at the time "rather than incur the wrath of an executive" he tried to "cover" the error.

"It was the wrong thing to do and I resigned," he told the inquiry.

But he said further allegations, published in the British Journalism Review, that a reporter the inquiry heard is believed to refer to Mahmood, offered "a financial bribe" to staff in the newspaper's computer room to make alterations to the copy, were "completely untrue".

During his evidence to the inquiry Mahmood said there was a "history of disagreements with one executive on the paper", in reference to Greenslade.

"This is a man who has written articles saying 'why I'm out to nail Mahmood'. He didn't like me then, he doesn't like me now".

Later in his evidence Mahmood was asked about the use of tipsters who had been defined as "unreliable witnesses" by police.

In response Mahmood said: "Most of people I deal with would be regarded as unreliable witnesses.

"But it's the information that's important, that we check thoroughly. It can't be said that because he had been described as unreliable by police that we thought he was unreliable.

"... We get stories from crack addicts, prostitutes ... our job is to test the tip. Only if lawyers are satisfied does it appear in the paper."